DE2009069A1 - Process for removing fluorine-containing impurities from air-diluted acidic exhaust gases - Google Patents

Description

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DIPL-CHEM-DR-EUSABETHJUNg a Munich 23 ,. DiPL-CHEM-DR-VOLKERVOSSIUS ίϋίΕϋΰΧΆ " * °".

DIPU.PHYS. DR. JÜRGEN SCHIRDEWAHnL SK ° ^{? ES8E} ''"« ντ / μομιΛιεν

PATENT LAWYERS

P 07 ¹ "C". February 26, 1970

SWISS ALUMINUM AQ Chlppis / Switzerland

"Process for removing contaminants containing fluorine air-diluted acidic exhaust gases ".

Priority; March 12, 1969 / Switzerland, registration no. 3709/69

The present invention relates to a method for removing fluorine-containing impurities from acidic, air-diluted exhaust gases. Such gases fall in the technical electrolytic aluminum production as "hall exhaust air", but also in the. Brick-laying industry, cement manufacturing and other chemical processes. In contrast to the corresponding non-air-diluted gases, which include, for example, the gas that is sucked off directly from the aluminum furnace at the point of origin, offers' the consistent rendering harmless and. Cleansing these gases diluted with large amounts of air pose major problems. A well-known water spray system, which is installed in the roof ridge of an electrolysis hall, removes hydrogen fluoride, sulphurous gases and the dust from the hall exhaust air, which consists of alumina and fluorides, in a satisfactory manner. This system consumes very large amounts of water, river water being sufficient. In some places these required amounts of water, including river water, are not available and one would therefore have to fall back on "tap water", i.e. spring water or other purified water: what teü ©? is una äie cost accounting su atark burdened © o

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The present invention has set itself the task of solving this problem of water shortage by purifying the water used for the absorption of the impurities and returning the purified water to the absorption system. Although it is known to be acidic. Neutralize and purify wastewater by adding Ca (OH) _{2 milk.} Nonetheless, it was not to be expected that a “lime cleaning ^{11 would be productive} without further ado for the special task in question, because as a result of the reaction of the COg and SO, content (or SO ₂ content) of the gases to be cleaned with the Usually excess calcium hydroxide added with disruptive excretion of CaCO ,, Ca sulfate and with disruptive accumulations of other substances such as NaCl, chlorine ions, was to be expected.

Surprisingly, it was nevertheless possible, according to the present invention, to solve the problem described ^{by means of a "lime cleaning 11, in which, according to the invention, certain conditions are met.}

The present invention consists in a method for removing F-containing and sulfur oxide and / or Cl-containing impurities from acidic, air-diluted exhaust gases by means of circulating, aqueous absorption liquid and for separating these impurities from the absorption liquid as compounds that are sparingly soluble in it. that one says © VtrvriVuLnigyr. ^ yi ν * :, ier Äbaorp ·

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absorbs tion liquid, then this liquid from the ' Absorption device leads away and a precipitation of its fluorine content together with a significant part of its other Impurities caused by combining with alkaline earth compounds, such as Ca (OH) O, with a final p "value of 5 - 8 being set, preferably from 5 * 5 - 6.5 * makes and the so formed Mash into a solid sludge containing the impurities by methods known per se (such as allowing it to settle) on the one hand and in the purified absorption liquid to be returned to the acidic absorption process on the other hand separates. The returned absorption liquid is, if it is not already set to acidic after cleaning is in the spray system in contact with the exhaust air as soon as possible angry*

There is nothing in the way and in some applications it is advantageous to use those contained in the absorption liquid undissolved dust ©) like clay, before the "Kalkreinigurig", ie Separately separated and recycled before the chemical precipitation of the fluorine * This separation can e.g. by sink separation (Settling) or centrifugation can be effected *.

According to an advantageous embodiment of the method According to the invention, prior to, during or after the precipitation of the alkaline earth metal fluoride of the solutions to be reacted, known per se Flocculants added, such as the chlorides and / or

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Sulphates of iron and / or aluminum, themselves a precipitate cause under the conditions of the inventive method. It is also advantageous to use precipitation aids known per se add that, like certain organic substances, e.g. polyacrylamide, the type and separability of the alkaline earth fluoride precipitate influence favorably.

According to a further embodiment, the fluorine can be precipitated as alkaline earth fluoride in a manner known per se in the presence of soluble alkaline earth salts such as CaCl ₂ , MgCl ₂ Acid contained in the absorption liquid is preferably not completely neutralized, but in any case no substantial alkalinity is brought about.

Another advantageous embodiment of the method of The invention consists in that part of the absorption liquid laden with fluorine compounds is removed from the absorption device, cleaned and the acidic absorption process

recirculates while another part of the loaded absorption liquid unpurified is returned to the acidic absorption process * One advantage of this embodiment is that it is so only has to enforce smaller amounts of loaded absorption liquid through the precipitation and separation plant and desnalb

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less system volume is required. In this case, too, can it is advantageous to separate the alumina-containing dust before the fluorine is precipitated

An advantage of the method of the invention is that with its help with the lowest water consumption fluorine-containing, strong air-diluted and therefore beautifully COp-containing and / or

with sulphurous gases such as SO ₂ * SO, mixed exhaust gases without '

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Every expulsion of contaminated liquids is rendered harmless can be. The water loss can be like that. on the evaporation loss and the loss of moisture from the sludge discharge can be limited. .

It is surprising that if the conditions of the method according to the invention - even while the Addition of chlorine ions and sulphate ions such as FeCl ,, Al-sulphate the cleaning precipitation - the. Content of the absorption liquid in these ions not at all or only insignificantly up to a certain point Equilibrium limit increases because these are soluble

Ions together with the alkaline earth fluoride as of the latter absorbed admixture are separated continuously before they appear disturbing. It is also surprising The further technical process effect of the process according to the invention is that the entire absorption liquid is here can recirculate completely and only has to replace the water lost through evaporation with fresh water.

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If one were to work with alkaline (e.g. NaOH) or with alkaline earth hydroxide-containing, circulated absorption liquids, as is known for the absorption and further processing of undiluted or slightly diluted F-containing exhaust gases, to solve the problem in question, this would result from the COg content of the exhaust gases and air, plenty of Na ₂ CO or CaCO, and this leads to solid crusts of Ca carbonate, sulfate and fluoride in the washing systems or spraying systems. These processes, which give rise to malfunctions and prevent the continuation of the relevant process, are intensified by the constant evaporation of water, which is carried away in gaseous form from the cleaned exhaust gases. In addition, these processes would cause additional consumption of NaOH or calcium hydroxide, which is affordable in terms of costs, the latter also occurring in the case of the regeneration of NaOH lye

by adding Ca (OH) ₂ to the NaF solution formed during absorption.

These disadvantages are avoided by the method according to the invention,

which at P ₁ , values of at most 8.0, preferably of η

at most 6.5 works, avoided

example

During the electrolysis of AlgO, at approx. 950 ⁰ C in the electrolysis furnace with fluxes consisting of NaF and AlF, an exhaust gas was produced which, in addition to solid, dusty compounds (mainly

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gO, and fluoride) contained hydrogen fluoride gas and acidic sulphurous gases such as SO ₂ , SO ₅ «The latter came from the carbon electrodes chemically converted during electrolysis ·

The exhaust gas mixed with hall air was washed with water as absorption liquid in a spray system installed in the roof ridge of the hall. Hydrogen fluoride, the sulphurous gases and the dusty compounds were absorbed by the water and the absorption liquid, measured at different times, increased to 50-10 mg F per liter, 150-250 mg SO |, per liter and the p "value of 3.5 was not reached. The absorption liquid discharged from the spray system was _{continuously combined with enough Ca (OH) 2} -MiICh that a p "value of 6.0 was achieved. In the present example, 5 liters of Ca (OH) ₂ milk with a content of 20 g of Ca (OH) liters were required per m of absorption liquid. As a precipitation

auxiliaries, 4 liters of iron chloride solution per wr to be cleaned absorption liquid were added and the precipitated

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Sludge was separated in a settling plant. At 105 its composition was dry: CaF ₂ : 35 % » ^A ^ p ° 3 ^: ^ ⁰ ^ * Ca (OH) ₂ : 0 %; CaSO ^: 4 Jg; CaCO,: 4 %; CaCl ₂ : 0.2 ^; SiO ₂ ? 5 * 5 ^ J.Fe (OH),: 2 & Cs 7.5 % and remainder H ₂ O.

*) (with a content of 2.5 g FeCU / liter)

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The aqueous absorption liquid ("purified water") separated from the precipitate was returned to the spray absorption system. It contained (measured at different times) in dissolved form:. _t . ' ,

. F '20-30 g / m ⁵
SO ^ 100 - 200 'g / m ⁵
Cl ¹ 50-150 ^

It was reused for the described purpose without disadvantage, like fresh water, without the cleaning effect of the spray absorption system being in any way noticeably weakened compared to the use of fresh water. An increase in the SO ₂ , "- and Cl 'content of the circulating absorption liquid to an extent that would have caused disturbances, took place despite the ongoing supply of sulphurous gases or Cl' ions (originating from the added ^ FeCl) , not.

Although both F-containing gases and exhaust air are cleaned by means of Water, as well as the separation of fluorine from fluoride solutions as a sparingly soluble fluoride compound such as CaFp and a cycle acidic gas scrubbing water was known per se separately for other applications, is the technology in the indicated possibility of the advantageous, consistently carried out and the special problem completely and smoothly dissolving combination according to the invention of these measures passed and the special process effects achieved according to the invention of enabling the exclusion of any fluorine repulsion

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containing water and avoiding disturbing accumulations has escaped her so far. This gives you the opportunity to achieve the same thing with very little water consumption so far has only been achieved with very high water consumption. .

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Claims (5)

- ίο - claims 1.) Process for removing fluorine-containing and sulfur oxide and / or chlorine-containing impurities from acidic, air-diluted exhaust gases by means of circulating aqueous absorption liquid and for separating these impurities from the absorption liquid as compounds that are sparingly soluble therein, characterized in that said impurities are in the absorption liquid absorbs, then the laden fluid is withdrawn from the absorption device and its Pluorgehalt including a substantial portion of its other impurities by the addition of alkaline earth metal, preferably Ca (OH ^, while adjusting a final p _H -value of 5-8, preferably from 5 , 5 - 6.5 fails and the mash formed in this way in per se known catfish into a die t
Contaminants containing pesticide sludge on the one hand and separates into the cleaned absorption liquid on the other hand, which is recycled to the absorption stage. 2.) The method according to claim 1, characterized in that before, during or after the alkaline earth fluoride precipitation, per se known flocculants, preferably chlorides and / or sulfates of iron and / or aluminum, and / or precipitation aids are added, preferably per se known organic substances, especially polyacrylamide. 009839/1892 3.) Method according to claim 1 and 2, characterized in that that the 'contained in the absorption liquid undissolved Dusts before the chemical precipitation of the fluorine is separated out on its own. 4.) Process according to claim 1 to 3-, characterized in that the fluorine content of the absorption liquid is precipitated _{in the presence of soluble alkaline earth salts, in particular CaCl 2} and / or MgCl _2. 5.) Method according to claim 1 to 4, characterized in that that part of the absorption liquid laden with fluorine compounds is withdrawn from the absorption device, cleaned and returns to the absorption stage, while another Part of the loaded absorption liquid unpurified returned * 009839/1892