EP1789156A1

EP1789156A1 - A process for recovering organic and inorganic components from a waste stream

Info

Publication number: EP1789156A1
Application number: EP05775475A
Authority: EP
Inventors: Geert-Jan Witkamp; Daniela Oana Trambitas
Original assignee: Kemira GrowHow Oyj
Current assignee: Yara Suomi Oy
Priority date: 2004-08-04
Filing date: 2005-08-03
Publication date: 2007-05-30
Also published as: FI20041056A0; WO2006013228A1; FI20041056A

Abstract

The present invention relates to a process for recovering an organic or inorganic acid, base and/or salt from an aqueous waste solution by cooling crystallisation, which waste solution further contains at least one organic component, comprising the steps of: a. precipitating said organic component by cooling the waste solution within a temperature range of 0 °C to a temperature below the freeze crystallisation point of said organic or inorganic acid, base or salt, b. removing the precipitated product of step (a) by separation from the waste solution, and c. precipitating the desired organic or inorganic acid, base or salt as crystals by cooling further the waste solution obtained from step (b) to the freeze crystallisation point of said organic or inorganic acid, base or salt, and d. recovering the precipitated product of step (c).

Description

A process for recovering organic and inorganic components from a waste stream

Field of the Invention

The present invention relates to a recovery process of inorganic or organic compounds in a form of mixed precipitates, preferably containing nitrogen, nitrates or nitric acid, from aqueous waste solutions containing additionally one or more organic components and optionally further inorganic components.

Background of the invention

For environmental and economical reasons there is a worldwide demand for reducing industrial, agricultural and communal waste streams and for recovering at least some of the components of the waste for recycling and reusing purposes. Conventionally i.a. evaporative crystallisation, cooling crystallisation, reverse osmosis, extractive crystallisation, and ion exchange have been used in the prior art for recovering desired components.

The treatment of the waste streams may however be problematic and laborious: Normally the waste streams are voluminous and the waste solution contains a complex composition comprising typically organic or inorganic acids, bases and/or salts desirable to be recovered, as well as minor amounts of further organic and/or inorganic components which are often difficult to separate from the desired product. Some of the components may also be hazardous. As a result the recovered product may still contain undesirable contaminants that can limit or even prevent the reuse of the recovered product.

For instance waste streams of nitric acid or a nitrate salt from industrial processes, fertilizer manufacturing industry, chemical industry, such as etching or explosive industry which are typically highly contaminated with inorganic components, such as heavy metals, and organic component(s), such as nitrochlorbenzene, caprolactame, guanide and nitric derivatives thereof. The separation of nitric acid i.a. from the organic components has proved to be difficult and has required e.g. a complete destruction of the organic matter using CO₂ and a mixture of concentrated HNO₃ and H₂SO₄ at elevated temperature. Recently, in WO 0128958 filed by the Applicant, a freeze crystallisation method has been disclosed for recovering crystalline material from an aqueous solution containing additionally organic contaminants. In said method the solution is cooled at a eutectic freezing point of the solution, whereby a flow of the crystalline material and a flow of ice crystals can be separated and removed simultaneously from the waste solution. The method is applicable for recovering e.g. salts and water from the waste streams. According to the document the organic components are contained in the remaining waste solution. In some cases, however, e.g. with high cooling rates or when the organic material has built up above a certain critical concentration (typically is 1-10%-w) in the waste stream, the organic material may contaminate the rapidly formed crystals as an impurity. Accordingly, there is a continuous need for more effective methods to recover and recycle components from waste streams for reducing the environmental pollution.

Moreover, there exists a need for economical methods for recovering desired products with improved purity.

Objective of the Invention

The object of the invention is to provide a process for the treatment of an mixed aqueous waste solution, which process enables an effective separation of different components from the waste solution.

Another object of the invention is to provide an industrially feasible and economical route for an efficient recovery of the components from waste streams. A further object of the invention is to provide a simplified and cost-effective process for recovering from aqueous waste solutions the desired inorganic or organic acid, base and/or salt product with improved purity levels.

Brief description of the drawings

Figure 1 shows the heavy metal content of the recovered nitric acid product of experiment 7 after the below described filtration and washing steps.

Figure 2 is a flow sheet of an example of a continuous process of the invention. Description of the invention

The invention is based on an unexpected finding that, when freeze crystallising inorganic or organic acids, bases and/or salts (below abbreviated as "acids, bases and/or salts") from their aqueous waste solutions containing at least one or more organic components as further dissolved component(s), said one or more organic components can also be precipitated by cooling as a separate mixed salt. This allows separating the organics from the solution prior to the simultaneous separation of salt and ice at eutectic point. The present invention can thus be used to decrease or prevent the contamination of the desired freeze crystallised acid, base or salt product(s) with said organic component(s) and, if desired, also for the recovery of organic components from the waste stream.

Accordingly, a process for recovering an organic or inorganic acid, base and/or salt from an aqueous waste solution by cooling crystallisation, which waste solution further contains at least one organic component, characterized by comprising the steps of: a. precipitating said organic component by cooling the waste solution within a temperature range of 0 ⁰C to a temperature below the freeze crystallisation point of said organic or inorganic acid, base or salt, b. removing the precipitated product of step (a) by separation from the waste solution, and c. precipitating the desired organic or inorganic acid, base or salt as crystals by cooling further the waste solution obtained from step

(b) to the freeze crystallisation point of said organic or inorganic acid, base or salt, and d. recovering the precipitated product of step (c).

In step (c) at least two products can be precipitated, in which case said products can be recovered successively or simultaneously in step (d).

Noticing the complex composition of the waste streams, it is surprising that also the "contaminating" organic component(s) can be precipitated and recovered separately, in a controlled manner, from a waste solution before the "actual" freeze crystallisation of the desired product(s) which are typically present in higher amounts in said waste solutions and desired to be recycled to the process. As to the used terms, in this application "the cooling crystallisation" means that the solution is cooled under 0 ⁰C to a temperature point, wherein the acid, base or salt to be recovered starts to crystallise mainly with ice crystals in the waste solution in question. Preferably, the solution is cooled to the eutectic point of the desired acid, base or salt of the solution, whereby, after said point is reached, both the crystallisation of water and the crystallisation of the acid, base or salt occur in the waste solution. In case of eutectic freeze crystallisation, either the ice crystals or the acid, base or salt crystals start to form before said eutectic point, depending i.a. on the concentration of the acid, base or salt in the solution. Eutectic freeze crystallisation is known in the art and has been disclosed and explained further e.g. by F. van der Ham et al in Chemical Engineering and Processing, 37, 1998, pp.207-213, and in WO 0128958, the contents of which are incorporated herein by reference. The ice precipitation and the acid, base or salt precipitation may be formed as separable phases in a known manner. Accordingly, also the water crystals can be recovered from the waste solution by gravity separation.

The process is applicable for the treatment of waste streams which contain dissolved an acid, base and/or salt that can be recovered by freeze crystallising and which additionally contain further dissolved component(s) comprising at least one or more organic components that can be precipitated when cooled under the conditions of the invention.

The waste stream to be treated can thus be any industrial, agricultural or communal aqueous waste solution, e.g. wastewater from fertiliser, food and milk product manufacturing processes, from chemical industry (such as acid catalysed chemical processes, chemical surface treatment, pickling, fine chemicals, pharmaceuticals), which contain said acid, base and/or salt and additionally organic material that can be precipitated according to the invention by cooling in suitable conditions. The acid, base and/or salt to be precipitated in step (c) from the waste solution is suitably an organic or inorganic acid and/or a salt thereof, preferably an organic or inorganic acid and/or a salt containing nitrogen, sulphur and/or phosphorus, hydrochloric acid, more preferably of nitric acid, sulphuric acid, phosphoric acid, formic acid and/or a salt thereof. In case of salts, the salt is preferably nitrate, sulphate and/or phosphate salt formed with ammonium or an element of Group 1 , Group 2, Group 11 or Group 14 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), preferably with sodium, potassium, magnesium, ammonium, copper, tin, silver and/or gold. As an example copper sulphate, ammonia and acetic acid may also be mentioned.

In one embodiment of the invention the waste solution contains at least a salt, preferably a salt of an acid, which is precipitated and recovered in step (c) and step (d).

According to one preferred embodiment the aqueous waste solution contains nitric acid and/or a salt thereof, which salt is preferably selected from one or more of the salts of the group of potassium nitrate, sodium nitrate, magnesium nitrate and ammonium nitrate, whereby either of acid or salt, or both, is precipitated and recovered in step (c) and step (d). In a further embodiment the waste solution contains nitric acid and optionally a salt thereof and the nitric acid is recovered from said solution.

The concentration of the acid, base and/or salt depends on the waste solution to be treated. Typically, the acid, base and/or salt to be recovered form(s) the main dissolved component(s) of the waste solution. Preferably said acid, base or salt, more preferably said acid or salt is the main component of the solution. For example the solution may contain at least 5 % by weight, preferably at least 10 % by weight of said acid or salt, such as of nitric acid and/or a salt thereof. As already mentioned the aqueous waste solution treated with the method of the invention contains dissolved one or more organic components which can be precipitated in step (a). Examples of the organic components include many of the room temperature ionic liquids, such as imidazolium, pyridinium derivatives, phosphonium, tetraalkylammonium compounds, guanidine and guanidine nitrate. Furthermore, said organic component(s) to be precipitated in step (a) may result from an industrial or agricultural process, i.e. be initially present in the waste solution, or may be a reaction product of a chemical reaction occurring in the waste solution e.g. between two or more components. In one preferable embodiment of the invention no reactants are added into the waste solution, but preferably two or more components initially present in the waste solution react together during the method of the invention, more preferably, when cooling the solution in step (a), whereby the reaction product thus formed precipitates in said step (a).

The concentration of the organic component(s) needed for the precipitation to occur depends typically e.g. on the aqueous waste stream to be treated, the concentration of the acid, base or salt to be precipitated and recovered in step (c) and step (d) and/or the freeze crystallisation temperature thereof and can be adjusted, if needed, by a person skilled in the art. As an example, the aqueous waste solution may contain for instance at least 1 % by weight of one or more organic components to be precipitated in step (a).

If needed, the organic component(s) of the waste solution can e.g. be concentrated in a pre-concentration step before step (a) e.g. to enable the precipitation of the organic component(s) to occur at the temperature range of step (a), or to accelerate or control the duration of the precipitation in step (a). The concentration step can be effected in a known manner e.g. by evaporative concentration and/or ultra-filtration. The concentration of the organic component(s) may also be increased by adding the mother liquid obtained from the step (d) of the present method to a fresh waste stream obtained from an industrial process, particularly when the recovery method is carried out continuously.

In step (a) of the method of the invention the cooling precipitation is effected within the range of 0 ⁰C to the freeze crystallisation temperature of the acid, base or salt to be recovered in the next precipitation step (c). Said freeze crystallisation point naturally depends i.a. on the composition and concentration of the components of the waste solution to be treated and can be determined e.g. experimentally e.g. for optimising the process. Such determination is within the ordinary skills of an artisan. The precipitation can be effected at one or more temperature points within said range. Suitably, the temperature can be decreased continuously during the step (a). The cooling rate is not critical and can be adjusted for each method for optimising the process.

In one preferred embodiment of step (a) the solution is cooled, preferably with a continuous cooling rate. The cooling rate may be varied e.g. from 0.02 to 1.5 °C/min, preferably from 0.1 to 1°C/min, and more preferably from 0.2 to 0.3°C/min. If desired the solution may be left and stirred for a period of time at a certain temperature point, e.g. at the lowest (end) temperature point used in step (a), to obtain the desired yield of the precipitate. It is also evident that the suitable temperature and precipitation time in step (a) may be chosen depending i.a. on the components of the waste solution and the concentrations thereof, e.g. the concentration of the organic component(s) to be precipitated, as well as on the desired yield of the precipitate. As an example the preferred cooling time ranges >2h at -30⁰C for step (a).

Alternatively, it is also possible to add a solvent to the waste solution, which solvent reduces the solubility of said organic component(s) in the formed mixture and thereby facilitates, e.g. accelerates, the precipitation during the cooling step (a). In the precipitation step (a) of the invention, the precipitation may occur due to decrease in solubility of the organic component(s), when the solution is cooled, or due to the cooling crystallisation or even due to eutectic freeze crystallisation. Furthermore, in step (a) one or more organic components may precipitate simultaneously or successively.

In the following step (b) the precipitate of step (a) is removed from the remaining waste solution. The removal step (b) can be carried out using any conventional separation method, such as filtration or decantation, preferably filtration. Step (b) can be effected after the step (a), or, alternatively, simultaneously during the precipitation step (a), whereby the precipitating product is separated from the solution directly after the formation thereof. The separated organic component(s) may be discarded or, if industrially valuable, also be recovered, whereby the precipitated product may optionally be further purified, e.g. washed, and recycled in the industrial process or reused for other industrial purposes.

Moreover, it was found that one or more inorganic components present in the solution as "impurities" which can disadvantageously contaminate the freeze crystallised product of the subsequent precipitation step (c), such as heavy metals, may at least partly be incorporated or bind to the organic precipitate in step (a). Accordingly, the purifying effect of step (a) on the freeze crystallisation product of step (c) may further be enhanced, since the waste solution remaining after step (a) typically contains also decreased amounts of these contaminating inorganic impurities, such as heavy metals.

If needed, the remaining waste solution may be concentrated before the precipitation step (c) e.g. for enabling or accelerating the precipitation of an acid, base or salt thereof.

In the actual freeze crystallisation step (c) the remaining waste solution obtained from step (b) is cooled to the freeze crystallisation temperature point of the acid, base or salt to cause the precipitation thereof. As already mentioned above, said point is preferably the eutectic freeze crystallisation point, whereby, in addition to the recoverable crystal precipitate, an ice crystal precipitate can also be obtained. In step (c) the solution is preferably cooled with a continuous cooling rate, preferably from 0.02 to 1.5°C/min, more preferably from 0.02 to 0.4°C/min, and most preferably from 0.1 to 0.4°C/min until the freeze crystallisation point is achieved. The cooling rate however is not critical and can be optimised for each process. Typically subcooling is required to initiate of the crystallisation of the desired acid, base or salt in the method of the invention, since the solution usually is still contaminated with other components, e.g. other acid, base and/or salt to be recovered after the freeze crystallisation of the first acid, base or salt, the organic components not precipitated in step (a) and/or the above mentioned impurities, compared to the freeze crystallisation temperature of e.g. a binary system of the desired acid, base or salt. Again, the temperature point and the precipitation time depend i.a. on the waste solution and the yield desired to recover. During the precipitation the temperature is kept in the freeze crystallisation point by cooling the system and optionally by removing the heat from the system. In practice, operating temperatures ranges are frequently below the theoretical values with 5-10 ⁰C. Furthermore, the aqueous waste solution may contain one or more of said acids, bases and/or salts to be precipitated and recovered at the steps (c) and (d). Then, after the crystallisation at the first temperature point, the cooling of the waste solution can be continued until the freeze crystallisation point of the next desired component is achieved for effecting the crystallisation thereof and then cooling is continued again to a lower temperature until all the desired products are crystallised. If desired, the eutectic freeze crystallisation equipment can deal even with the simultaneously recovery (by precipitation) of more than one components.

In step (d) the crystallised recoverable product of step (c) is removed from the remaining solution. The removal can be effected e.g. by decanting or filtrating in a known manner. In case of a eutectic freeze crystallisation, the acid, base or salt crystals and the ice crystals are preferably removed separately e.g. utilising the density difference between said two crystal products. As the precipitation and removal steps (a) and (b), also the precipitation and removal steps (c) and (d) can be effected successively or simultaneously in the same step.

The recovered product crystals are optionally washed and preferably melted to obtain the desired product for further use.

With the present process the obtained freeze crystallised product has advantageously improved purity compared to product crystallised from a solution that has not been treated according to step (a) of the present invention.

The process of the invention is very suitable for recovering nitric acid from waste streams which contain organic components, since due to the present invention the organic component(s) can be separated more effectively.

Furthermore, the method was found to be especially advantageous for recovering nitric acid from waste streams which contain guanidine and/or guanidine nitrate as the organic component(s). Particularly, it was found that when said nitric acid containing waste solution was cooled according to step (a) as described above, said guanidine and guanidine nitrate surprisingly precipitated as nitroguanidine in the nitric acid waste solution. Nitroguanidine obtained with the present method is known to be a very valuable raw material e.g. in medical and chemical industry, such as in the preparation of sulfa drugs, explosives, rocket propellant, guanidine carbonate or other valuable derivatives of guanidine, paints, photo materials, disinfectants etc. Thus after step (a), the formed nitroguanidine precipitate can be removed from the solution according to step (b) as described above, optionally washed and/or further purified for resuse e.g. for the above purposes. The present method provides a highly feasible means for producing nitroguanidine during the recovery process of nitric acid from guanidine and/or guanidine nitrate containing waste stream. Accordingly, one preferred embodiment of the invention provides a process for recovering nitric acid from a waste stream which contains nitric acid, one or more organic components, which are precipitable in step (a), e.g. iminourea derivatives, guanidine and guanidine nitrate, preferably guanidine and/or guanidine nitrate, and optionally further components, typically inorganic impurities, e.g. heavy metals. Preferably the amount of nitric acid is at least 5 % by weight, preferably at least 10 % by weight, and more preferably at least 30 % by weight. The amount of said organic component(s), preferably of guanidine and/or guanidine nitrate, is at least 2 % by weight. Such waste streams are typically obtained from industry, e.g. surface treatment, dyes, medicine, in the manufacture of explosives (such as nitroglycerine), nuclear wastes. The solution may also contain one or more salts of the nitric acid.

At the precipitation step (a) guanidine is preferably cooled between 0-(-3O) ⁰C. The temperature can vary depending on the temperatures needed at step (c). E.g. depending on the hydration forms, there are different nitric acid eutectic temperatures e.g. -45⁰C, -42°C or even -70⁰C.

Nitric acid is preferably freeze crystallised as nitric acid trihydrate.

The treatment of an aqueous waste solution according to the method of the invention as described above is also provided. The process of the invention, i.a. the precipitation step (a) and the freeze crystallisation step (c) may be effected in any systems which allow the sufficient cooling and, in case of an eutectic freeze crystallisation, preferably the separation of the recoverable crystal flow and the ice flow. If desired both steps can be effected in the same device. Such devices has been disclosed in the prior art, e.g. in said WO 0128958 or in said article, Chem. Eng. Proα, 37, 1998, pp. 207-213.

The process of the invention can be carried out batch wise or, preferably, continuously. In a continuous process fresh waste stream may be fed as such or after any pre-treatment step, such as after pre-concentration, to a process line and the steps (a)-(d) and any optional further steps, such as a further concentration step, may be effected e.g. sequentially in the process line. Moreover, the remaining waste solution obtained from step (d) can, if desired, be recycled in the process as such, or after a further treatment step, by combining it with the fresh waste stream feed. As an example the process line may contain one or more cooling units, such as disclosed above, and one or more filtering/washing units which may be provided with a separate cooling system, as well as optional concentration units. Such units are well known in the art.

The invention thus provides a simplified, time- and cost-saving tool to recover several components from a waste stream for the reuse purposes.

Figure 2 is a flow sheet depicting a continuous process of the present invention. The waste solution is fed as a feedstock to the cooling and precipitation step (a). Thereafter the precipitated organic product is separated from the waste solution in a solid-liquid separator (step (b)). The partly purified waste solution is then fed to the cooling/freeze crystallisation step (c) followed by recovering the crystallised product(s) in a second solid- liquid separator (step (d)). The separated crystals are subsequently washed with a washing liquid. The remaining waste solution leaving the second solid-liquid separator can be discharged together with the washing liquid from the crystal washing or recycled to the cooling/freeze crystallisation step (C).

The percentages in this specification are % by weight unless otherwise specified.

Examples

In the following experiments 1-15 an aqueous waste solution obtained from the industrial preparation process of nitroglycerine or from different surface treatment (pickling industries) was used to demonstrate the recovery method of the present invention.

The waste solution contained about 53-54 weight % HNO₃ and, as further components, organic material which comprised mainly guanidine and guanidine nitrate in an amount of more than 2 weight-% as well as heavy metal impurities. In the precipitation step (a) guanidine and/or guanidine nitrate was precipitated and recovered as nitro guanidine and in the precipitation step (c) nitric acid was recovered as nitric acid tri-hydrate crystals (HNO₃3H2O).

In each experiment 300 ml waste solution was placed in a 500 ml metal double wall jacket bath crystalliser coated with Halar (E-CTFE, ethylene- chloro trifluoroethylene copolymer 1 :1 alternated, trademark Halar). The crystalliser was provided with a cooling unit and a stirrer with adjustable stirring rate. Methanol was used as the cooling medium.

The solution was stirred with a constant stirring rate during the cooling and precipitation steps (a) and (c).

The cooling rates for precipitating the organic material and nitric acid were varied. The precipitation step (a) of the invention: The waste solution was cooled with the given cooling rates between 0.15 and 1.02 °C/min as indicated in table 1 , whereby formation of the precipitate observed in the solution after the temperature dropped below 0 ⁰C. The precipitated product was nitro guanidine, which was formed from guanidine and guanidine nitrate components in the nitric acid in the presence of sulphuric acid when cooling the solution. The amount of the precipitate increased during the further cooling as most probably the product's solubility decreases with temperature. The cooling step (a) was continued until the temperature was decreased from -1 to -40 ⁰C. The end point of the cooling step (a) and the precipitated amount of nitro guanidine are indicated in table 1. During cooling ice was formed as the temperature dropped below -1°C and was removed separately.

Table 1 : Precipitation step (a) of the invention for recovering the nitro guanidine product.

* left over night at -3O⁰C

The nitroguanidine precipitate was white, fine and solid at the room temperature, whereby it could be easily filtered at room temperature and washed.

In general, the amount of the precipitated nitroguanidine in a nitric acid waste solution containing guanidine or guanidine nitrate depends i.a. on the cooling rate, end temperatures and the duration of step (a), and can be adjusted accordingly noticing e.g. the desired levels to be recovered and/or to be removed from the remaining solution.

The amounts of inorganic impurities present in the initial waste solution, in the remaining solution after removing the nitro guanidine precipitate and in the nitro guanidine precipitate of step (a) were determined for in the experiment 7 and are given in table 2.

Table 2. Impurity levels as determined in experiment 7 for the initial waste solution, the precipitate of step (a) and the remaining solution after step (a)

The results of table 2 show that the amount of the heavy metal impurities in the remaining waste solution can advantageously be decreased, and thus the contamination of the precipitated product of the following step (c) be reduced or eliminated, due to the incorporation of at least part of the impurities into the precipitate of step (a).

After the removal of nitro guanidine the remaining waste solution was returned to the crystalliser and the cooling procedure was continued with continuous stirring.

Due to the high amounts of impurities present in the initial nitric acid waste solution, impurities were also present in the solution after step (a). Therefore a significant under-cooling temperature was needed in all experiments (down to -47 to -49 ⁰C) to initiate the precipitation of the nitric acid crystals tri-hydrate. Furthermore, in all experiments, the initial phase of the crystallisation process was indicated by a sudden increase of the tempe- rature to a value between -28 to -32 ⁰C. The crystallisation occurred rapidly, in 10 to 20 seconds. The nitric acid was crystallised in the form of a nitric acid tri-hydrate crystals (HNO₃*3H₂O), which were needle-like shaped and stable between -30^°C and -4O⁰C.

The nitric acid crystals were separated from the mother liquor by filtration at -30 °C in a double wall filtration funnel with glass pores and provided with a cooling unit. The crystals of experiment 6 were further washed in said filtration unit at -30 ⁰C using a pure 69 weight % nitric acid solution doped with a "tracer" (Lanthanium) and pre-cooled at -30 ⁰C for the purity determinations of the recovered product. The filtrated and optionally washed crystals where then melted to obtain the recovered nitric acid solution

(53.8w-%HNO₃).

Figure 1 shows the heavy metal content of the recovered nitric acid product of experiment 7 after the above described filtration and washing steps.

The cooling rates together with the initial crystallisation temperature of the nitric acid and the temperature increase at the beginning of the crystallisation of the nitric acid at this precipitation step (c) are indicated in table 3. Table 3: Precipitation step (c) of the invention for recovering nitric acid as crystals

Claims

1. A process for recovering an organic or inorganic acid, base and/or salt from an aqueous waste solution by cooling crystallisation, which waste solution further contains at least one organic component, characterized by comprising the steps of: a. precipitating said organic component by cooling the waste solution within a temperature range of 0 ⁰C to a temperature below the freeze crystallisa¬ tion point of said organic or inorganic acid, base or salt, b. removing the precipitated product of step (a) by separation from the waste solution, and c. precipitating the desired organic or inorganic acid, base or salt as crystals by cooling further the waste solution obtained from step (b) to the freeze crystallisation point of said organic or inorganic acid, base or salt, and d. recovering the precipitated product of step (c).

2. The process of claim 1 , wherein at least two products are precipitated in step (c), and said products are recovered successively or simultaneously in step (d).

3. The process of claim 1 or 2, wherein in step (a) the solution is cooled with a continuous cooling rate, preferably from 0.02 to 1.5°C/min, more preferably from 0.1 to 1°C/min, and most preferably from 0.2 to 0.3^°C/min.

4. The process of any of the preceding claims, wherein the precipitated product of step (a) is separated by filtration or decantation from the waste solution and optionally recovered for reuse.

5. The process of any of the preceding claims, wherein in step (c) the waste solution is cooled to the eutectic freeze crystallisation temperature point, after which point crystals of said organic or inorganic acid, base or salt and ice crystals are formed.

6. The process of claim 5, wherein at step (c) the precipitated acid, base and/or salt crystals and the ice crystals are removed separately from the waste solution.

7. The process of any of the preceding claims, wherein in step (c) the solution is cooled with a continuous cooling rate, preferably from 0.02 to 1.5°C/min, more preferably from 0.02 to 0.4°C/min, and most preferably from 0.1 to 0.4°C/min to the freeze crystallisation point.

8. The process of any of the preceding claims, wherein the waste solution is preconcentrated before the precipitation step (a).

9. The process of any of the preceding claims, wherein the waste solution contains said organic or inorganic acid, base or salt at least 5 % by weight.

10. The process of any of the preceding claims, wherein the waste solution contains said organic component at least 1 % by weight.

11. The process of any of the preceding claims, wherein the waste solution is an agricultural waste stream or a waste stream from chemical industry, such as from a fertiliser, explosive manufacturing industry, chemical surface treatment, fine chemicals, pickling industry, pharmaceuticals.

12. The process of any of the preceding claims, wherein said organic or inorganic acid, base or salt is nitric acid, sulphuric acid, phosphoric acid, formic acid and/or a salt thereof.

13. The process of claim 12, wherein the salt is a nitrate, sulphate and/or phosphate salt formed with sodium, potassium, magnesium, ammonium, copper, tin, silver and/or gold.

14. The process of any of the preceding claims, wherein nitric acid and/or a salt thereof, preferably potassium nitrate, sodium nitrate, magnesium nitrate and/or ammonium nitrate, is recovered at step (d).

15. The process of any of the preceding claims, wherein nitric acid is crystallised at all eutectic points of nitric acids corresponding to the hydrated forms in step (c).

16. The process of any of the preceding claims, wherein the organic component is selected typically from guanidine and/or guanidine nitrate.

17. The process of claim 16, wherein the content of guanidine and/or guanidine nitrate is between 1-10 % by weight.

18. The process of any of the preceding claims, wherein a waste solution containing nitric acid and guanidine or guanidine nitrate is cooled in step (a), whereby a nitro guanidine product precipitates from the waste solution.

19. The process of claim 18, wherein the waste solution is cooled between 0-(-3O) ⁰C during the precipitation step (a).

20. A process for the treatment of a wastewater using a method of any of claims 1-19.