FI79474B - FOERFARANDE FOER FRAMSTAELLNING AV EN VATTENLOESNING SOM INNEHAOLLER KALIUMBIKARBONAT OCH KALIUMKARBONAT OCH SOM LAEMPAR SIG FOER TVAETT AV SURA GASER. - Google Patents

Description

79474

Process for the preparation of an aqueous solution containing potassium bicarbonate and potassium carbonate suitable for washing acid gases

The invention relates to a process according to the preamble of claim 1 for preparing an aqueous solution containing potassium bicarbonate and potassium carbonate suitable for washing acids containing gaseous compounds, such as industrial exhaust gases.

According to such a method, an aqueous solution of a suitable potassium compound is carbonized, after which the aqueous phase containing potassium bicarbonate and carbonate is separated from the reaction mixture.

The solution prepared by the invention can be used for the purification of industrial exhaust gases, whereby potassium sulphates and nitrates are obtained from the potassium chloride used as starting materials and, for example, from gases containing sulfur dioxide and nitric oxide.

Acid gases, such as flue gas scrubbing solutions, should be of sufficient time to neutralize the gases. At the same time, it is desirable that the reaction products formed have at least some utility value. Today, e.g. the following time reagents: ammonia water, sodium sulfite, aluminate or carbonate, potassium sulfite, phosphate or acetate. However, none of these is completely satisfactory in all respects.

The use of Na carbonate and Na hydroxide gives sodium sulphate, which is a worthless waste.

Potassium compounds are used e.g. K2SO3 and a mixture of K2CO3-Na2CO3. The downside of these methods is e.g. the fact that the sulfite-bisulfite-SO2 reaction pushes the equilibrium in the reaction 2SC> 2 + K2SO3 + H2O -> 2KHSO3 to the left.

The Atomic International process has to use high temperatures and pressures, which in turn causes 2,779,474 severe corrosion problems.

The use of potassium bicarbonate would provide useful products, such as potassium sulphates and nitrates, which could be used, for example, as fertilizers. Reactions of potassium carbonate with sulfur and nitrogen oxides give similar products, and aqueous potassium carbonate is more alkaline than aqueous potassium bicarbonate. The combination of potassium bicarbonate and potassium carbonate would thus give a sufficiently alkaline washing solution which otherwise meets the above requirements.

Potassium bicarbonate is usually made from potassium carbonate by introducing carbon dioxide into a saturated solution of potassium carbonate or by contacting moist potassium carbonate with carbon dioxide gas at moderate pressures. Potassium carbonate is again obtained by carbonizing electrolytically prepared potassium hydroxide. Alternatively, a so-called Engel process has been used, in which carbon dioxide gas is introduced under pressure into a solution of neutral magnesium carbonate or oxide and potassium chloride or sulphate, after which the potassium carbonate is recovered by decomposing the double salt formed.

However, none of these methods offer an economically interesting alternative for the preparation of industrial exhaust scrubber.

Blumberg, R. et al. (Proceedings of the International Solvent Extraction Conference 1974) have disclosed a process for preparing sodium bicarbonate from NaCl by liquid extraction.

The reactions are then: 3 79474

NaCl (aq) + CO 2 (g) + «2 ° + '“ (o) ^ NaHCO 3 (s) +

AmHCl, v (c)

AmHCl, + + MM (OH) * MgCl9 + Am + Ho0 C °} 2 (s) * 2 (s) 2 iMgCl, + H90 * Mg (0H), + HCl, .2 (s) 2 2 (s) (acl> where Am is a water-insoluble primary, secondary or tertiary amine, and aq denotes the aqueous phase, g the gas phase, o the organic phase and s the solid phase.

The method of Blumberg et al. Is well suited for the selective preparation of sodium bicarbonate. Because the solubility of sodium bicarbonate in water is relatively low, about 16.4 g / L at 60 ° C, sodium bicarbonate precipitates from the aqueous phase and is easily isolated.

The authors do not suggest the application of the method to the preparation of potassium bicarbonate, nor is it very well suited to the preparation of pure potassium bicarbonate. Potassium bicarbonate is highly soluble in water, which is why it would be considerably laborious and time-consuming to separate it from the aqueous phase.

The method of Blumberg et al. Further uses magnesium hydroxide to regenerate the amine to give dilute hydrochloric acid. Removing this from a reasonably concentrated process requires high CO2 pressures, which in turn causes e.g. corrosion risks.

It is an object of the present invention to provide a novel process for the preparation of potassium bicarbonate and potassium carbonate solutions suitable for the recovery of acidic compounds.

The invention uses potassium chloride as a starting material, which is dissolved in water. The carbonation is carried out by liquid extraction, 4 79474, whereby an aqueous solution of potassium chloride is contacted with a water-immiscible organic solvent used as the extraction solvent, which contains the reaction product of an organic amine and carbon dioxide, amine carbonate. The potassium bicarbonate formed is not isolated from the aqueous solution, so it also partially decomposes into potassium carbonate, carbon dioxide and water.

By keeping the aqueous phase at a suitably dilute potassium ion concentration, both potassium bicarbonate and potassium carbonate are maintained in solution. The pH of the potassium bicarbonate-carbonate solution is about 9 and it is excellently suitable e.g. For scrubbing gases containing N0X and SO2 ~. In this case, the nitrogen oxides are also bound as a potassium salt in a useful form.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

The reaction sequence according to the invention is described by the following equations: (1) KCl (aq) + Am (O) + CO2 + H2O -> KHCO3 (gq) + AmHCl2) (2) 2KHCO3 (aq) -> K2CO3 (aqj + H2O + CO2)

In the reaction, the amine is regenerated (3) for a new extraction cycle (1) with calcium hydroxide.

(3) AmHCl (O) + fcCa (OH) 2 -> Am (O) + ^ CaCl2 The potassium chloride used as starting material is per se a well-known substance. Its solubility in water is 56.7 g / 100 g H2O at 60 ° C. The properties and preparation of potassium chloride have been described in the literature, cf. e.g., Encyclopedia of Chemical Technology, Kirk-Othmer (1967) 388

The amine used in the process for the preparation of potassium bicarbonate may be, for example, a secondary amine such as diisononyl

II

5 79474 amine (DINA) or a product marketed under the trademark "Amberlite LA-2". Its general formula is then

R 1 is NH 2 in which R 1 and R 2 independently represent a straight or branched alkyl group having 1 to 10 carbon atoms, and the molecular weight of the amine is preferably 270 to 290.

As the amine, it is also possible to use a tertiary amine of the formula R 2 to NH, R 3 in which R 1, R 2 and R 3 independently represent an alkyl group having 1 to 10 carbon atoms. One preferred tertiary amine is trioctylamine of the formula: C8H13 C8H13- ^ NH c8h13 ^

The amine is dissolved in an anhydrous solvent having a sufficiently high boiling point, preferably above 120 ° C. Suitable solvents are aliphatic hydrocarbons or aromatic hydrocarbons, such as toluene and xylene.

The introduction of carbon dioxide gas into the amine solution gives an amine bicarbonate of the formula according to the amine used is either 6 79474 R 1 -NH 2 HCO 3 (sec-amyl) R 1 or R 1 -NH-NH 3 CO 3 (tert-amyl)

Extraction of the potassium salt solution with amine bicarbonate gives potassium bicarbonate (potassium bicarbonate), leaving the amine hydrochloride in the extraction solvent. The amine is regenerated with a suitable alkali, such as quicklime or slaked lime (CaO or Ca (OH) 2) / magnesium oxide, hydroxide or carbonate, respectively, and either ammonia water, alkali hydroxide or carbonate.

Potassium bicarbonate decomposes to some extent in potassium carbonate solution.

An aqueous solution of potassium bicarbonate and potassium carbonate prepared as described above can be used for scrubbing industrial exhaust gases, flue gases, etc. Potassium bicarbonate reacts with sulfur dioxide to form potassium sulfite, potassium sulfate is formed in the reaction between potassium bicarbonate and sulfur trioxide, and potassium nitrate or nitrite, respectively, is obtained from the reaction of potassium bicarbonate and various nitrogen oxides.

The invention has considerable advantages. Thus, a readily available rock salt can be used as a starting material. The washing ability of the obtained washing solution is better than that of the K2SO3-KHSO3 solution. Nitrogen oxides are recovered in the wash as nitrates and are suitable as a fertilizer in addition to the potassium sulphate obtained in the process.

The invention will now be described in more detail with the aid of the accompanying drawing, which shows in principle a flow diagram of the process according to the invention.

7 79474

The potassium salt used in the process is fed to tank 3.1.2, to which water is simultaneously fed to dissolve the potassium salt.

The amine carbonate is in turn prepared in a mixing vessel 3.4 by adding carbon dioxide to the amine solution, whereby the reaction 4 (4) Am (o) + CO 2 (g) + H 2 O (g) -> AmHCO 3 (o) takes place.

The potassium salt solution is filtered in a filter apparatus 3.2 and fed through an intermediate tank 3.1.3 to a mixer-settler-type extraction column 3.3.5. Amine carbonate is fed simultaneously to this, whereupon reaction 5 (5) takes place in the extraction column 3.3.5 AmHCO 3 (O) + KCl (aq) + H 2 O -> AmHCl (0) + KHCO 3 (aq)

When it is desired to achieve a yield of about 99% in the extraction, 2-10 extraction steps are required. By using a sufficiently high phase ratio, preferably Eo / Ro = 1.5-4, particularly preferably 2-3, the number of extraction steps can be kept to five. The phase ratio indicator Eo denotes the extraction solvent flow and the denominator Ro the aqueous phase flow, respectively.

For extraction, the extraction solvent leaving the extraction is fed to a tank 3.7 equipped with a stirrer, in which the amine is regenerated, e.g. with slaked lime.

Process conditions vary; in extraction the temperature is about 40 ... 50 ° C, in regeneration 60 ... 80 ° C. The pressure in the extraction is less than 0.5 aty.

The aqueous solution leaving the extraction, mainly containing KHCO3, is now fed via intermediate tank 2.5 to scrubber 1.4, where the gases containing SO2, SO3 and NOx are washed with an aqueous solution of KHCO3. In this case, e.g. the following reactions: 8 79474 (6) KHCO. + SO. -> ^ K.SO. + CO. + H.O,.

3 (aq) 2 (g) 2 3 (aq) 2 (g) 2

(7) KHCO. + SO. - »* K SO, + CO. + H.O, N

. 3 (aq) 3 <g) 2 \ aq) 2 (g) 2 <*) (8) KHCO. + N.O. -> KNO. + CO. + Ηο0, ·.

J (aq) 2 5 (g) 3 (aq) 2 (g) 2 <8) (9) KHCO. + NO. -> KNO. + CO. + H.O,, J (aq) 2 (g) 2 (aq) 2 (g) 2

The washing solution is circulated in the scrubber through the tanks 1.4.1.

The scrubber 1.4 used in the method can be of any usual type, i.e. e.g. an empty tower with spray or spray nozzles at its upper end or filled with filling pieces. The scrubber can also be of the type in which spacers are arranged in the tower, by means of which sudden changes in the direction of the gas flow, which enhance the transfer of matter, are effected. One preferred type of scrubber is the so-called venturi scrubber operating on the co-current principle.

The aqueous solution K2SO3-K2SO4-KN02 leaving the scrubber can now be evaporated and crystallized in the evaporation and crystallization equipment 2.1 to 2.4. The crystalline mass obtained after separation of the crystals is dried and the K2SO3 and KNO2 in it are oxidized to K2SO4 and Kili by blowing air into the dryer.

Example 1:

In the example case, the potassium bicarbonate-potassium carbonate solution to be used was prepared as follows: 6.3 kg of potassium chloride were fed per hour into tank 3.1.2, giving a water supply of 21.2 kg / h. In the extraction column, an amine solution, 9,79,474 prepared by introducing carbon dioxide gas into a 2-molar toluene solution of trioctylamine, was extracted at 40 ° C with potassium chloride solution. A 4 molar aqueous solution of potassium bicarbonate was then obtained in about 99% yield. The toluene solution of the amine was regenerated with calcium hydroxide at 60 ° C and recycled to the mixing vessel 3.4.

If necessary, the prepared potassium bicarbonate solution can be diluted with water, e.g. to about 2 molar, in which case it contains about 200 g of KHCO 3 / l.

Use of potassium bicarbonate solution for flue gas cleaning:

The flue gas of the coal-fired power plant, which contained 3000 mg / m3 SO2 and had a temperature of 140 ° C, was passed to a venturi scrubber 1.4, where a potassium bicarbonate solution with a concentration of 200 g KHCO3 / I flowed.

The flow rates were: - gas 50,000 m3 / h - washing solution 10 m3 / h.

The sulfur content of the gas leaving the scrubber was less than 140 mg / m3. The solution from the scrubber now contained potassium sulfite, nitrite and nitrate.

The concentrations of the washing solution after a sufficient circulation time (approx. 9 h) were: - K +: 80 g / l - SO3: 816 g / l

The crystals were separated and the potassium sulfite and nitrite were oxidized to potassium sulfate and nitrate, respectively.

Comparative Example:

The following example compares the preparation of a bicarbonate compound from sodium chloride and, according to the invention, from potassium chloride.

A solution of diisononylamine (DINA) in toluene was prepared. The concentration of the solution was 500 g dina / l.

10 79474

The dina was now carbonized by introducing CO 2 gas into it for 15 minutes with constant stirring.

A solution of 95 g / l was then prepared from NaCl. This solution was now shaken with the dinicarbonate solution prepared above.

the phase ratio eo / ro was 2: 1 (eo organic phase, ro = water phase). Shaking time 15 min., Clarification time 30 min.

Analysis results: pH Cl- g / 1 ro 6.8 57.1 rr 8.4 5.2 rr leaving aqueous phase A 4-m solution was prepared from KCl and shaken as above with dinicarbonate.

Analysis results: pH Cl- g / 1 ro 6.2 139.9 rr 8.7 20.2 In the latter experiment, rr was clear, i.e. all the KHCO3 formed had dissolved in the aqueous phase. Since the solubility of NaHCO 3 is only about 40 g / l, part of the formed NaHCO 3 precipitated in its own step.

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Claims (8)

A process for the preparation of an aqueous solution containing potassium bicarbonate and optionally potassium carbonate suitable for the washing of acid gases, in particular industrial flue gases, according to which process - carbonating an aqueous solution of a suitable potassium compound and then separating the aqueous phase containing potassium bicarbonate and carbonate characterized in that - as a starting material, potassium chloride is dissolved in water; contains the reaction product of an organic amine and carbon dioxide. Process according to claim 1, characterized in that an organic solvent is used, the boiling point of which is above 120 ° C. 3. A process according to claim 2, characterized in that an aromatic hydrocarbon is used as an organic solvent. Process according to claim 3, characterized in that toluene is used. Process according to claim 1, characterized in that as the reaction product, the reaction product of carbon dioxide and a secondary amine of the general formula is used.