GB2315434A - Purifying waste gas from fertiliser production - Google Patents

Abstract

In a process and an apparatus for the joint purification in a wash column (VII) by means of a washing liquid (5) of the waste gases (1,3) and/or vapors (2) formed in the production of inorganic fertilizers, the waste gases and/or vapors are supplied to the wash column in at least two separate streams (1-3). One of the streams is a collective waste gas stream which contains the waste gases and/or vapors with a high water content and high proportion of ammonia. The other stream is a collective waste gas stream with a low water content and low content of ammonia. Apparatus for the purification is typically a filled column with at least two supply lines for waste gas/vapours connected to separate gas distributors in the wash column.

Description

Purification of the waste gases formed in the production of inorganic fertilizers The invention relates to a process and an apparatus for the joint purification of the waste gases and/or vapors formed in the production of inorganic fertilizers.

Processes for the production of inorganic fertilizers, such as nitrophosphate (NP) or nitrophosphate-potassium (NPK), in general have the following sequential process steps: - Digestion: The dissolution of the solid raw materials (crude phosphate, diammonium phosphate, monoammonium phosphate, etc.) in mineral acids and removal of insoluble constituents - Transformation: Specific crystallization of undesired constituents - Neutralization: Neutralization of the acidic solution with NH3 - Evaporation: Concentration of the solution, also described as a mash, by evaporation of water - Mixing with potash: Metering-in of potash and other salts for formula adjustment - Granulation and drying: Shaping of the mash into granules and residual water removal by means of combustion gasesihot air.

In these process steps, waste gases and vapors (furzes) are formed in different amounts with different contents of nitric oxides (NOx), ammonia (NH3), water vapor, hydrogen fluoride (HF), silicon tetrafluoride (SiF4) and solid dusts (alkali metal and alkaline earth metal nitrates, sulfates, phosphates, fluorides and mixed salts), depending on the raw materials employed and the final formula of the fertilizer.

According to the legal regulations, the waste gases and vapors must be treated before release to the environment and the substance contents must be reduced. A number of techniques are known in which individual or, in some cases, collected waste gases are purified in one or more stages: Thus EP-A 0 440 932 describes a process for the washing of waste gases from fertilizer production, which contain ammonia and dust. The wash solution is a 50% strength ammonium nitrate solution acidified with nitric acid. A formula (inequation) for the calculation of the pH is mentioned, which guarantees that no ammonium nitrate aerosols are formed. The pH has an effect exclusively on aerosol formation. It has no effect on the washing action with respect to dust.

Furthermore, US 4,662,929 describes a multistage process for the purification of the waste gases from digestion, neutralization and mixing with potash on the one hand in an at least two-stage spray wash and for the purification of the waste gases from granulation and drying on the other hand in an apparatus sequence consisting of cyclone, spray washer and solids separator. Both work-ups are coupled to one another via the wash solution sequentially running through all spray washers.

EP-A 0 562 328 describes the joint processing of the collected waste gases and vapors from neutralization, evaporation, mixing with potash, granulation and drying in an only one-stage apparatus having separatory fittings. The waste gases are purified and simultaneously, by means of the heat content of the waste gases, sufficient water is evaporated such that solids concentrations of dissolved and undissolved constituents of up to 60% by weight are achieved in the wash solution. The wash solution originates from the fertilizer process and is fed back into it. It contains additives (e.g. nitric acid and/or ammonia), phosphate ions and the substances originating from the process and separated from the waste gases.

In all processes in which volatile bases, e.g. ammonia, and acids, e.g.

nitric acid, and water vapor meet in the gas phase, the formation of aerosols cannot generally be prevented. Aerosols are very small droplets which, together with the gas, pass through the wash column and are not separated. This aerosol formation in general occurs in complex chemical and hydrodynamic systems. If aerosols occur to a significant extent, the legally prescribed harmful substance thresholds are well exceeded. In order to prevent emission into the environment, additional aerosol separators are installed. Alternatively, gas streams which each mainly contain ammonia or acid vapors are purified separately, as is described in US 4,662,929.

It can be taken from the prior art that customarily various substances contained in the waste gases are removed one after the other from the waste gas streams in individual steps and using different wash solutions.

Aerosol formation here generally is causally not prevented. The simultaneous treatment of collected waste gases in a single-stage washing apparatus, as is described in EP-A 0 562 328, also leads here to the large number of substances contained in the waste gases, in particular ammonia and mineral acid vapors, forming aerosols by reaction with one another. The measure described in EP-A 0 440 932 of the adjustment of a minimum pH has shown no effect in the complex chemical field in the case of collective waste gas treatment.

It is an object of the present invention largely to avoid aerosol formation in the purification of the total waste gases formed in the production of inorganic fertilizers in a wash column.

This object is achieved in a process for the joint purification in a wash column by means of a washing liquid of the waste gases and/or vapors formed in the production of inorganic fertilizers. The inventive process comprises supplying the waste gases and/or vapors to the wash column in at least two separate streams.

According to a particularly advantageous embodiment of the invention, one of the streams is a collective waste gas stream which contains the waste gases and/or vapors with a high water vapor content and high proportion of ammonia (NH3), and a further separate stream is a collective waste gas stream with a low water content and low content of ammonia (NH3). This further collective waste gas stream can essentially consist of the waste gases from granulation and drying.

Surprisingly, it has been found that using the procedure according to the invention, i.e. if the final mixing of all waste gas collective streams is only carried out in the wash column, aerosol formation can be almost completely avoided. In the purified waste gases of the wash columns operated in this way, the legal thresholds were consequently adhered to and it was no longer possible to detect aerosols.

The separate streams of collective waste gases are led into the wash column by means of individual distributor systems and distributed there.

The distributor systems distribute the streams under the fittings arranged in the wash column, the stream with a low water and ammonia content being distributed in the wash column above the first stream with a high water and ammonia content.

The abovementioned process steps can be particularly advantageously carried out using an apparatus in which the waste gases and/or vapors formed in the production of inorganic fertilizers are carried out in a wash column provided with fittings by means of a washing liquid, in which the wash column has at least two separate supply lines for the supply of the collective waste gases to be purified, which are connected to separate distributors in the wash column. In a known manner, the fittings in the wash column can be filling materials, packings or plates. The distributors are arranged below these fittings in the wash column. In this case, the distributor for gases with a low water an NH3 content can be designed as a vantetype gas distributor and the distributor for gases with a high water and NH3 content can be designed as a tubular distributor with drillings.

The distributor for gases with a low water and NH3 content is in this case advantageously arranged above the distributor for gases with a high water and NH3 content. The tubular distributor is advantageously divided into at least two tubes preferably Nnning in a V-shape after supply to the wash column.

Further details and advantages can be taken from the process described in the following with the aid of the working example shown in the drawing and the apparatus shown.

Fig. 1 shows a scheme of the process according to the invention in block form; Fig. 2 shows a schematic representation of the wash column employed for carrying out the process, partly in section; Fig. 3 shows a perspective cross section through the lower part of the wash column shown in Fig. 2.

In the process scheme shown in Fig. 1, the known sequential process steps mentioned at the outset are shown in block form and provided with Roman reference symbols. In these stages, the following process steps take place: I. Digestion: Here the solid raw materials (crude phosphate, diammonium phosphate, monoammonium phosphate, etc.) are dissolved in nitric acid, and the insoluble constituents are removed.

II. Transformation: Here calcium nitrate tetrahydrate is separated off by specific crystallization. In this process, an acid phosphate solution is formed which essentially consists of a mixture of nitric and phosphoric acid.

m. Neutralization: In this stage, the acidic, enriched phosphate solution is neutralized with ammonia. Water evaporates here due to the heat of reaction.

IV. Evaporation: The solution (mash) is concentrated in this stage by further evaporation of water.

V. Mixing with potash: Potassium and other salts are metered in here to adjust the final formula of the product to be prepared.

VI. Granulation and drying: In this stage, the shaping of the mash into granules and the removal of residual water by means of hot air is carried out.

From this sequence of process stages are formed individual gas streams which are supplied to the lower region of a wash column VII provided with fittings. The wash column VII can be designed as a packed column.

It can also be equipped with packings or plates in a known manner. The branched streams are specifically the streams designated by Arabic numerals 1, 2 and 3. The waste gas formed in the transformation in step H, which essentially contains NOX and fluorides as harmful substances, is supplied by line 1 to a gas distributor IX arranged under the fittings of the wash column VII. Together with the gas passed through line 1 into the gas distributor Ix, the waste gas from the granulation and drying (VI) is also supplied to this through line 3. From the neutralization (m), the evaporation (IV) and from the mixing with potash (V), vapors and fumes are led off through the lines 2 and supplied to a distributor X lying under the gas distributor IX. The water vapor-containing waste gases supplied to the distributor X essentially contain NH3 and fluorides.

Both distributors are described in detail later with the aid of Figures 2 and 3.

The bottom liquid obtained in the wash column VII is led off through a line 5 and fed back into the upper part of the wash column VII by means of a recirculating pump VIII. From the wash suspension fed into the circulation through line 5, which also contains undissolved solids, concentrated wash suspension is led out through line 9 and fed back into the production process of the inorganic fertilizer. To compensate for water losses and to compensate for the concentrated wash suspension removed through line 9, process and/or rinsing water are supplied through line 7.

Ammonia or nitric acid can be metered into these through line 8 in order to adjust the pH of the water supplied through line 7 to the pH of the circulation suspension in line 5. To adjust the pH and/or the phosphorus concentration of the wash solution fed into the circulation in line 5, it is possible to add through line 6 nitric acid or acidic phosphate solution, which can be removed via the line 4 between the stages transformation (II) and neutralization (m). If necessary, phosphoric acid can also be supplied. The purified waste gas is removed through line 10.

The wash column VII shown schematically in Fig. 1 will now be explained in greater detail with the aid of Fig. 2.

This wash column VII is essentially a conventional wash column of cylindrical structural design, in whose upper section fittings VIIA, preferably filling materials, are arranged. Above this is provided a distributor XI for the wash liquid supplied through line 5. Above the flange XII, the column VII is connected to the waste gas line 10. Its collecting tube Xm arranged in the lower part is connected via the line 5 to the recirculating pump VIII.

Essential to the invention are the gas distributor IX arranged under the fittings VIIA and the distributor X for the vapors and fumes arranged under this, whose details are described in the following with the aid of Fig. 3.

The distributor IX for the collective waste gas stream with a low water content and low content of ammonia, which is supplied through the lines 1 and 3, is in this working example a vane-type gas distributor which has impact surfaces IXA, against which the collective gas stream flowing in through the flange-shaped opening MB flows and is distributed. At the same time, this gas stream is exposed to the washing liquid flowing out through the fittings VIVA, whereby impurities are dissolved and dusts are washed down. The vane-shaped impact surfaces IXA are held in a fixed position in struts IXC, which in the working example run together conically. The angle of the impact surfaces with respect to the gas stream flowing in through the opening IXB is fixed depending on its amount and speed.

Below the vane-type gas distributor IX is arranged a tubular distributor X which is provided with drillings XA. In the working example, the gas distributor X consists of two tubes led parallel to the gas distributor, through whose supply openings XB is supplied the gas stream 2 which contains the waste gases and/or vapors with a high water content and high proportion of ammonia. The drillings XA are arranged such that they are preferably directed outward diagonally to the longitudinal axis of the wash column. The distributor for gases with a high water and NH3 content can also be designed as a nozzle, where one or more nozzles can be provided. The axis of the jet direction of these nozzles preferably runs at right angles to the axis of the wash column.

In a process operating according to the scheme described in Fig. 1 with a column according to Figs. 2 and 3, the results summarized in the following tables were achieved. Here the wash solution used is a process rinsing water treated with NH3, in which the pH is adjusted with 60% strength HNO3. The gases 1 and 3 in this case together as "dry waste gases" form the further collective waste gas stream, which is characterized by a low water vapor content and low ammonia content.

The gas 1 additionally contains a high content of NOX. This collective waste gas stream is passed into the wash column VII via the vane-type gas distributor IX.

The gas 2, which has a high water vapor content and high ammonia content and which can be described as "moist waste gas" is passed into the wash column VII through the tubular distributor X provided with drillings XA.

From the amount of ammonia nitrogen present in the purified waste gas "pure gas 10", the theoretically maximum possible amount of ammonium nitrate aerosols was back calculated.

Gas streams Gas 1 Gas 2 Gas 3 Pure gase 10 after washing Amount (m /h) 1320 22,000 305,000 333,000 Temperature ( C) 24 85 88 56 Moisture (g H2O/kg 16.5 412.0 55.5 90 NOx measured (mg/m ) 830 10.5 4.1 11.6 Ammonia nitrogen 1.7 293.6 90.5 1.6 measured (NH3 N) (mg/m ) Ammonium nitrate 9.1 aerosols calculated from NH3 N (mg/m ) Liquid stream Process HNO3 Wash solution Removal water 60 % strength Amount (m /h) 4.4 0.125 Trickling 0.6 densily 35 m31m2h Temperature ( C) 25 25 56 56 pH 5.1 3.6 3.6

It can be taken from the experimental results obtained that, using the procedure according to the invention of the final mixing of the waste gas collective streams, aerosol formation can be almost completely avoided only in the washing apparatus. Thus, the known problem, that in bringing together ammonia and nitric acid vapors and NOx large amounts of aerosols of ammonium nitrate customarily form which cannot be separated in a wet wash, are surprisingly largely avoided.

Claims (17)

1. A process for the joint purification in a wash column by means of a washing liquid of waste gases and/or vapors formed in the production of inorganic fertilizers, which comprises supplying the waste gases and/or vapors to the wash column in at least two separate streams.

2. A process according to claim 1, wherein one of said streams is a collective waste gas stream which contains the waste gases and/or vapors having a high water vapor content and a high proportion of ammonia (N H3) and wherein a further one of said streams is a collective waste gas stream having a low water content and a low content of ammonia (NH).

3. A process according to claim 2, wherein the said further one of said gas streams essentially consists of waste gases from granulation and drying.

4. A process according to any one of claims 1 to 3, wherein said separate streams are distributed by means of individual distributor systems in the wash column.

5. A process as claimed in claim 4 when dependent on claim 2 or 3, wherein the distributor systems distribute the streams under fittings arranged in the wash column, the said further one of said streams being distributed above the first stream in the wash column.

6. An apparatus for the joint purification by means of a washing liquid of waste gases and/or vapors formed in the production of inorganic fertilizers, comprising a wash column (VII) provided with filling means (VIIA), e.g. filling or packing materials, and at least two separate supply lines for the supply of the collective waste gases to be purified which are connected to separate distributors (IX,X) in the wash column.

7. An apparatus according to claim 6, wherein the distributors (IX,X) are arranged below the filling means (VIIA) in the wash column (VII).

8. An apparatus according to claim 6 or 7, wherein at least one distributor (IX) is provided for gases with a low water and NH3 content and at least one distributor (X) is provided for gases with high water and NH3 content.

9. An apparatus according to claim 8, wherein the at least one distributor (IX) for gases with a low water NH3 content is arranged above the at least one distributor (X) for gases with a high water and NH; content.

10. An apparatus according to claim 8 or 9, wherein the at least one distributor (IX) for gases with a low water and NH3 content comprises vane-type gas distributors (IXA,IXB).

11. An apparatus according to claim 8, 9 or 10, wherein the at least one distributor (X) for gases with a high water and NH3 content comprise tubular distributors with holes, e.g. drillings, (XA, XB) therein.

12. An apparatus according to claim 11, wherein at least two tubular distributors having separate supply lines are provided.

13. An apparatus according to claim 11 or 12, wherein the axes of the holes in the tubular distributors are disposed substantially at right angles to the axis of the wash column.

14. An apparatus according to claim 8 or 9, wherein the at least one distributor for gases with a high water and NH3 content have at least one nozzle.

15. An apparatus according to claim 14, wherein the said nozzles have jet axes disposed substantially at right angles to the axis of the wash column.

16. A process for the joint purification in a wash column by means of a washing liquid of waste gases and/or vapors, the process being substantially as hereinbefore described with reference to, and as illustrated in, Figures 1 to 3 of the accompany drawings.

17. An apparatus for the joint purification of a washing liquid constructed and arranged substantially as herein described with reference to, and as illustrated in, Figures 1 to 3 of the accompanying drawings.