Title of Invention

A PROCESS FOR THE JOINT PURIFICATION OF THE WASTE GASES FORMED IN THE PRODUCTION OF INORGANIC FERTILIZERS

Abstract

A process for the joint purification of the waste gases formed in the production of inorganic fertilizers The present invention relates to a process for the joint purification by means of a wash solution in a wash column of the \. aste gases formed in the production of inorganic fertilizers in which the process waters form at least one part of the wash solution, wherein the wash solution has a phosphate content of at least 1% by weight, preferably of at least 2 % by weight, calculated as P2O5, dissolved in the clear wash solution, the pH of the wash solution is set in a range from pH 3 to pH 5.5, preferably pH 3.6 to 4.2, by means of mineral acids, preferably nitric acid (HNO3) and the circulated wash solution is at least 25 m /m h, based on the cross-sectional area of the washing apparatus.

Full Text

The invention relates to a process for the joint purification by means of a wash solution of the waste gases formed in the production of inorganic fertilizers. Process waters are customarily described as aqueous solutions which e.g. are formed in the production process of the inorganic fertilizers, and those which are formed on rinsing apparatus parts, as well as wash solutions which originate from waste gas washing of other apparatuses. These waste waters can additionally be diluted in order not to exceed specific concen¬trations of chemicals; they are not always clear solutions, i.e. they comprise often undissolved solids, too. Processes for the production of inorganic fertilizers such as nitrophosphate or nitrophosphate-potassium fertilizers (NP or NPK fertilizers) 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: Addition of potash and other salts to adjust the formula Granulation and drying: Shaping of the mash into granules and residual water removal by means of combustion gases/hot air. When carrying out these process steps, waste gases and vapors occur in different amounts and with different contents of nitric oxides (NOx), ammo¬nia (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 inorganic fertilizer to be produced. According to the existing legal regulations, the waste gases and the vapors must be treated before release to the environment in order to reduce the ingredients contained. A number of processes for this have been published. Thus EP-B 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 calculating the pH is described which gua¬rantees that no ammonia nitrate aerosols are formed. The pH exclusively affects the aerosol formation. It has no effect on the washing action with respect to dust. US 4,342,731 describes a process for the purification of combustion furnace waste gases, which contain hydrogen sulfide and ammonia. In this process, an apparatus is alternatively operated either as a purification column or as an evaporation apparatus. The apparatus is divided into various zones (spatially separate apparatus sections) and is described as a "concentrator/-washer". EP-A 0 562 328 describes the joint working-up of the collected waste gases and vapors from neutralization, evaporation, mixing with potash, granulation and drying in only an apparatus with fittings having separatory activity, in which a thermodynamic stage has been accomplished. The waste gases are purified, and at the same time by means of the heat content of the waste gases sufficient water is evaporated such that a solids concentration (dis¬solved and undissolved constituents) of up to 60% by weight can be achie¬ved in the wash solution. The process waters for the completion of the wash solution originate from the fertilizer process, and the concentrated suspension 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. The publications mentioned describe processes in which only individual waste gas streams from fertilizer production are purified and/or individual harmful substances are removed from the waste gases only selectively and/or multistage processes are provided for the purification. The simultaneous treatment of collected waste gases in a wash apparatus having an accom¬plished thermodynamic stage, as is described in EP-A 0 562 328, leads to the large number of harmful substances and solids contained in the waste gases being precipitated in the wash solution and lead there by reaction with one another and precipitation/crystallization to insoluble sediments and baked-on materials in the liquid distributor of the circulating wash solution and in the fittings having separatory activity (e.g. fillings, packings of filling materials). This results in significantly reduced service lives with frequent operational breakdowns, shutdowns and cleanings. It is an object of the present invention significantly to reduce, in a process for the purification of the waste gases formed in the production of inorganic fertilizers, using process waters in a purification column, the operational breakdowns, shutdowns and cleaning cycles caused by deposits in the washing apparatus. This object is achieved by a process for the joint purification by means of a wash solution in a wash column of the waste gases formed in the produc¬tion of inorganic fertilizers in which the process waters form at least one part of the wash solution. The inventire process comprises the wash solution having a phosphate content of at least 1% by weight, preferably of at least 2% by weight, calculated as P205, dissolved in the clear wash solution. A clear wash solution is defined here as the wash solution without the undis¬solved solids normally contained in it. According to a particularly advantage¬ous embodiment of the invention, the phosphate content of the wash solution is adjusted by use of the phosphate-containing dusts formed in the produc¬tion process of the inorganic fertilizers and/or acidic phosphate solution. The dusts contained in the waste gas in the production process comprise, depen¬ding on the type of inorganic fertilizer produced, different amounts of phosphates. Their precipitation during the washing process contributes to the P205 concentration adjustment. The amount of P205 formed by means of the dusts can be so great that an additional supply of the acidic phosphate solution, which is formed by crystallization in the transformation mentioned at the outset, is not necessary. Metering-in of H3P04 for phosphate adjust¬ment is likewise possible, but usually not necessary. According to a particularly advantageous embodiment of the invention, the pH of the wash solution is set in a range from pH 3 to 5.5, preferably pH 3.6 to 4.2, by means mineral acids, preferably by means of nitric acid (HN03). As wash solution, advantageously acidic process and rinsing waters formed in the production process of the fertilizers are used, which are. likewise adjusted to the abovementioned pH before introduction into the washing apparatus by introduction and dissolution of NH3 or HN03. According to an advantageous development of the invention, the circulated wash solution is at least 25 m3/m2h, based on the cross-sectional area of the washing apparatus. On suitable choice of fillings, the amount circulated can be up to 125 m3/m2h. Accordingly the present invention provides a process for the joint purification by means of a wash solution in a wash column of the waste gases formed in the production of inorganic fertilizers in which the process waters form at least one part of the wash solution, wherein the wash solution has a phosphate content of at least 1% by weight, preferably of at least 2 % by weight, calculated as P2O5, dissolved in the clear wash solution, the pH of the wash solution is set in a range from pH 3 to pH 5.5, preferably pH 3.6 to 4.2, by means of mineral acids, preferably nitric acid (HNO3) and the circulated wash solution is at least 25 m3/m2h, based on the cross-sectional area of the washing apparatus. Further details and advantages of the invention can be taken from the working example described below with the aid of the process scheme shown in the drawing. In this process scheme, 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: Calcium nitrate tetrahydrate is separated off here by specific crystallization. An acidic phosphate solution is formed in this process, which essentially consists of a mixture of nitric and phosphoric acid. III. Neutralization: In this stage, the acid-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: To adjust the final formula of the product to be produced, potassium and other salts are metered in here. 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 takes I place. Individual gas streams are formed in this sequence of process stages, which are supplied to the lower area of a washing column VII provided with fittings, preferably with filling materials. In detail, these are the following streams designated by Arabic numerals 1, 2 and 3. The waste gas formed in conversion in step II, which essentially contains NOx and fluorides as harmful substances, is supplied to the wash column VII through line 1. The wash column VII can be designed as appacked column. It can also be equipped with packings or plates in a known manner. From the neutraliza¬tion (III), the evaporation (IV) and from the mixing with potash (V), vapors and fumes are led off through the lines 2 and supplied to the wash column VII. They are water vapor-containing waste gases which essentially contain the following compounds: NH3, fluorides. They can be passed on together and fed to the wash column VII together with the waste gas passed through line 1. The waste gas from the granulation and drying (VI) is additionally introduced through line 3 into the wash column VII in this feed line. The bottom liquid formed in the wash column is drawn off through a line 5 and supplied by means of a recirculating pump VIII to the upper part of the wash column VII. From the wash suspension, which also contains undis¬ solved solids, circulated through line 5, concentrated wash suspension is passed out through line 9 and fed back into the production process of the inorganic fertilizer. To compensate for the water losses and to compensate for the concentrated wash suspension drawn off through line 9, process and/or rinsing waters 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. For the adjustment of the pH and/or the phosphate concentration of the wash solution circulated in line 5, it is possible to add through line 6 nitric acid or acidic phosphate solution, which can be drawn off through line 4 between the stages transformation (II) and neutralization (III). If necessary, J phosphoric acid can also be supplied. The purified waste gas is drawn off through line 10. The results achieved with an apparatus operating by the scheme described above can be taken from the following tables. The gases supplied to the wash column are provided here with the Arabic numerals used in the description of the scheme. The operating data for a collective waste gas wash according to the invention in a single-stage packed column are summa¬rized in the tables. pH adjustment was carried out with 60% strength HN03. It was found that on keeping to the operating parameters according to the invention the service life of the wash column was increased very significant¬ly compared with known processes, such as described, for example, in EP-A 0 562 328. Sedimentations and baked-on materials are severely inhibi¬ted and reduced while at the same time keeping to the legally prescribed waste gas threshold values. This is accompanied by a significant reduction in the costs for shutdowns, loss of production and cleaning expenditure. WE CLAIM 1. A process for the joint purification by means of a wash solution in a wash column of the waste gases formed in the production of inorganic fertilizers in which the process waters form at least one part of the wash solution, wherein the wash solution has a phosphate content of at least 1% by weight, preferably of at least 2 % by weight, calculated as P2O5, dissolved in the clear wash solution, the pH of the wash solution is set in a range from pH 3 to pH 5.5, preferably pH 3.6 to 4.2, by means of mineral acids, preferably nitric acid (HNO3) and the circulated wash solution is at least 25 m7m h, based on the cross-sectional area of the washing apparatus. 2. The process as claimed in claim 1, wherein as wash solution, acidic process and rinsing waters formed in the production process of the inorganic fertilizers are used, which are adjusted to the necessary pH by introduction and dissolution of NH3 or HNO3. 3. The process as claimed in claim 1 or 2, wherein, the phosphate content of the wash solution is adjusted by absorption of the phosphate containing dusts formed in the production process of the inorganic fertilizers. 4. The process as claimed in any one of claims 1 to 3, wherein the phosphate content of the wash solution is adjusted by controlled metering-in of the acidic phosphate solution formed in the production process of the inorganic fertilizers. 5. The process as claimed in any one of claims 1 to 4, wherein the phosphate content of the wash solution is adjusted by controlled metering-in of H3PO4. 6. A process for the joint purification by means of a wash solution in a wash column of the waste gases formed in the production of inorganic fertilizers substantially as herein described and exemplified with reference to the accompanying drawings.

Documents:

1621-mas-1997 abstract duplicate.pdf

1621-mas-1997 abstract.pdf

1621-mas-1997 claims duplicate.pdf

1621-mas-1997 claims.pdf

1621-mas-1997 correspondence others.pdf

1621-mas-1997 correspondence po.pdf

1621-mas-1997 description (complete) duplicate.pdf

1621-mas-1997 description (complete).pdf

1621-mas-1997 drawing duplicate.pdf

1621-mas-1997 drawing.pdf

1621-mas-1997 form-19.pdf

1621-mas-1997 form-2.pdf

1621-mas-1997 form-26.pdf

1621-mas-1997 form-4.pdf

1621-mas-1997 form-6.pdf

1621-mas-1997 others.pdf

1621-mas-1997 petition.pdf