Title of Invention	A PROCESS FOR THE COMBINED PURIFICATION OF THE WASTE GASES
Abstract	The present invention relates to a procesws for the combined purification of the waste gases,formed in the preparation of mineral fertilizerts,by means of a wash solution in a scrubbing column in which process waters form at leats part of the wash solution,an additive being added to the wash solution,wherein the additive introduced is a polymer of acrylic acid and/or of methacylic acid.

Full Text

Combined purification of the waste gases, formed in the preparation of mineral fertilizers, by means of a wash solution rhe present invention relates to a process for the combined purification of the waste gases, formed in the preparation of mineral fertilizers, by means of a wash solution in a scrubbing column in which process waters form at least part of the wash solution, an additive being added to the kvash solution. Process waters are usually defined as aqueous solutions tfhich form in the process for the preparation of mineral fertilizers, and hose which are obtained during rinsing of parts of apparatuses, including vash solutions which originate from the scrubbing of waste gases in other ipparatuses. These waste waters may be dilute in order to avoid exceeding certain concentrations of chemicals. 'rocesses for the preparation of mineral fertilizers, such as nitrophosphate >r nitrophosphate-potassium fertilizers (NP or NPK fertilizers), generally lave the following sequential process steps: Digestion: dissolution of the solid raw materials (crude phosphate, diammonium phosphate, monoammonium phosphate, etc.) in mineral acids and removal of insoluble components Conversion: controlled crystallization of calcium nitrate Neutralization: neutralization of the acidic solution with NH3 Evaporation: concentration of the solution, also referred to as a slurry, by evaporation of water Admixing of potassium: metering in of potassium and other salts for establishing the formula Granulation and drying: shaping of the slurry into granules and removal of residual water by means of combustion gases/hot air. These process steps give rise to waste gases and vapors in different amounts and with different contents of oxides of nitrogen (NOx), ammonia (NH3), water vapor, hydrogen fluoride (HF), silicon fluoride (SiF4) and solid dusts (alkali metal and alkaline earth metal nitrates, sulfates, phosphates and fluorides and mixed salts), depending on the raw materials used and on the final formula of the mineral fertilizer to be produced. According to the existing legal regulations, the waste gases and vapors must be treated before being released into the environment, in order to reduce the ingredients present. A number of processes have been disclosed for this purpose. Thus, EP-Bl 0 440 932 describes a process for scrubbing 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 (inequality) for calculating the pH, which ensures that no ammonium nitrate aerosols are formed, is claimed. The pH influences exclusively the formation of aerosols. It has no effect on the scrubbing action with respect to dust, EP-A3 0 562 238 describes the combined working up of the collected waste gases and vapors from neutralization, evaporation, admixing of .potassium, granulation and drying in only one apparatus containing packings having a separation effect, which has realized a thermodynamic stage. The waste gases are purified, and at the same time water is evaporated, by means of the heat content of the waste gases, in an amount such that a solids concentration (dissolved and undissolved components) of up to 60% by weight can be reached in the wash solution. The process waters for supplementing the wash solution originate from the fertilizer production process, and the concentrated suspension is recycled to said process. It contains additives (for example, nitric acid and/or ammonia), phosphate ions and the substances originating from the process and separated off from the waste gases. As a result of the simultaneous treatment of collected waste gases in a scrubbing apparatus with a realized thermodynamic stage, the plurality of harmful substances and solids contained in the waste gases are precipitated in the wash solution and lead there, by reaction with one another and precipitation/crystallization, to insoluble sediments and cake materials in the liquid distributor of the circulating wash solution and in the packings having a separation effect (for example, dumped or stacked packing). This results in considerably shortened times onstream with frequent operational faults, shutdowns and cleaning operations. BE 865 351 has therefore also envisaged reducing incrustations in the wash liquor of stack gas scrubbers by means of additives. The additives envisaged here were phosphonates, esters of polysulfonic acid with higher alcohols and polyols, acrylate polymers and polyacrylamides in concentrations of from 5 to 50 g/m , which corresponds to about 5 - 50 ppm. However, these are solutions of comparatively low concentration. Finally, EP 538 026 Al discloses the use of copolymers of acrylic acid with aminoethylenephosphonic acid/aminoethylenephosphonates for the prevention of calcium sulfate deposits in the high-temperature oxidation of metal ores dissolved in sulfuric acid. These are extremely acidic solutions in which the solubility with respect to the critical salts is very high. It is an object of the present invention to reduce the amount of, or entirely avoid, deposits in the scrubbing of waste gases, as formed in the preparation of mineral fertilizers, and hence to increase the availability of the plants. We have found that this object is achieved, according to the invention, if the additive introduced is a polymer of acrylic acid and/or of mcthacrylic acid. In a particularly advantageous embodiment of the invention, the additives used are low molecular weight polyacrylic acids and alkali metal salts thereof having molecular weights of from 1,000 to 20,000. Advantageously, polyacrylic acids terminated with secondary alcohols, in particular with isopropanol or secondary butanol, are used here. Particularly advantageous additives are low molecular weight polyacrylic acid terminated with isopropanol and having a molecular weight of from about 2,000 to 8,000, preferably from about 3,000 to 5,000, and sodium salts thereof. They are prepared by free radical polymerization of acrylic acid and methacrylates in secondary alcohols or in mixtures with other solvents, preferably water, at least 10% of the solvent consisting of secondary alcohols. The amount of the additive is set such that concentrations of from 100 to 1,000 ppm, preferably from 200 to 400 ppm, based on the mass of the wash solution, are present in the wash solution. Sodium polymers effective according to the invention are commercially available, for example, as Good Rite K752 from Goodrich (USA), Dispex N40 from Allied Colloids (GB) and Sokalan CP10 from BASF (D). We have found that the use of a novel additive leads to substantially delayed crust formation in wash suspensions in the scrubbing of waste gases from fertilizer production. Compared with previous one- and two-weekly cleaning of the waste gas scrubbing column, times onstream of at least 8 weeks are achieved with the use of the novel process. The embodiment of the novel process which is described below reveals further details and advantages of the present invention. EXAMPLE A scrubbing column as described in EP-A3 0 562 238 and equipped with packings is fed, via a line, with process water from fertilizer production, by means of which collected waste gases from the fertilizer production are purified. The circulating wash suspension was composed of the following main components: undissolved) 38.3% In order to investigate the mode of action of the additive used according to the invention, the suspension was heated at 60°C in a stirred container and was stirred. A 5.4% strength ammonium sulfate solution and an 11% strength calcium nitrate solution were metered in simultaneously in order to simulate the further increase of crystal-forming ions. This corresponds to the concentration increase in the scrubbing of waste gases as a result of the dusts washed out. The reference used was an experiment without the introduction of an additive. The results with the use of the additive SOKALAN* CP 10 are summarized in the Table below: 10 on crust formation. The efficiency increases with increasing concentration. A concentration of from 200 to 400 ppm was found to be sufficient for use in the waste gas purification process. The generally known sensitivity of additives to high alkaline earth metal concentrations, with flocculation and precipitation of the polymer, was tested in a separate experiment. For this purpose, 1000 ppm of SOKALAN* CP 10 was added to the abovementioned wash suspension and the latter was stirred for one hour at about 60°C. Here, as in the precipitation experiments shown in the above Table, no polymer flocculation was observed. We claim:- 1. A process for the combined purification of the waste gases, formed in the preparation of mineral fertilizers, by means of a wash solution in a scrubbing column in which process waters form at least part of the wash solution, an additive being added to the wash solution, wherein the additive introduced is a polymer of acrylic acid and/or of methacrylic acid. 2. A process as claimed in claim 1, wherein the additives used are low molecular weight polyacrylic acids and alkali metal salts thereof having molecular weights of from 1,000 to 20,000. 3. A process as claimed in claim 2, wherein the additives used are polyacrylic acids terminated with secondary alcohols, in particular with isopropanol or secondary butanol. 4. A process as claimed in claim 1, wherein the additives used are low molecular weight polyacrylic acid terminated with isopropanol and having a molecular weight of from about 2,000 to 8,000, preferably from about 3,000 to 5,000, and the sodium salts thereof. 5. A process as claimed in any of the preceding claims, wherein the amount of the additive is set such that concentrations of from 100 to 1,000 ppm, preferably from 200 to 400 ppm, based on the mass of the wash solution, are present in the wash solution. 6. A process for the combined purification of the waste gases, formed in the preparation of mineral fertilizers by means of a wash solution, substantially as herein descr ibed, and exempli f ied.

Documents:

mas-1997-1846-abstract.pdf

mas-1997-1846-claims duplicate.pdf

mas-1997-1846-claims original.pdf

mas-1997-1846-correspondance others.pdf

mas-1997-1846-correspondance po .pdf

mas-1997-1846-description complete duplicate.pdf

mas-1997-1846-description complete original.pdf

mas-1997-1846-form 1.pdf

mas-1997-1846-form 26.pdf

mas-1997-1846-form 3.pdf

mas-1997-1846-form 4.pdf