RU2807933C1 - Method for cleaning flue gases of calcination furnaces - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: used in processes where fuel is burned to produce combustion products containing CO₂, in particular in the process of producing alumina (Al₂O₃) from aluminum hydroxide (Al(OH)₃) in calcination furnaces. A method for purifying flue gases from a calcination furnace in the production of alumina includes (a) supplying flue gases from the calcination furnace sequentially to a multicyclone and an electric precipitator to purify said gases from dust. This is followed by (b) supply of purified gases for wet cleaning into the scrubber. Then (c) irrigation of the gases in the scrubber with a calcination filtrate containing caustic soda with a concentration of at least 16 g/l to obtain a pulp. And then (d) separation of the pulp into liquid and solid phases. In this case, the liquid phase is re-fed to the irrigation stage (c), and the solid phase is returned to the alumina production process at the charge sintering stage.

EFFECT: increasing the degree of purification of the exhaust flue gases of calcination furnaces from dust and reducing the level of CO₂ in the exhaust flue gases.

2 cl, 1 ex, 1 tbl, 1 dwg

Description

Field of technology to which the invention relates

The invention relates to metallurgy and can be used in processes where fuel is burned to produce combustion products containing CO ₂ , in particular in the process of producing alumina (Al ₂ O ₃ ) from aluminum hydroxide (Al(OH) ₃ ) in calcination furnaces.

State of the art

In the process of producing alumina (aluminum oxide), the aluminum hydroxide pulp is fed to filtration to remove the liquid phase (calcination filtrate), the resulting cake enters the calcination furnaces, in which natural gas is burned to maintain the required temperature to produce alumina. The flue gases leaving the furnaces are cleaned of dust particles and released into the atmosphere.

There is a known method of calcination, according to which wet aluminum hydroxide is sent to a calcination furnace, calcined and cooled; Flue gases from the furnace are cleaned of the dust they contain and released into the atmosphere. Dust cleaning is carried out sequentially in multicyclones and electric precipitators (Troitsky I.A. et al. Metallurgy of aluminum. - M.: Metallurgy, 1977, p. 111).

The technical solution is known according to patent RU No. 2125016, publ. 01/20/1999, according to which the method of calcination of aluminum hydroxide includes feeding wet aluminum hydroxide into the flue, heat treating it in suspension with flue gases leaving the furnace, which are previously cleaned of dust, calcining the aluminum hydroxide and cooling the alumina, while the dust is cooled and removed from recycling. Disadvantages of the above known methods:

– absence of a stage for post-purification of flue gases from dust - wet cleaning in a scrubber, which allows you to capture the smallest dust after the electric precipitator;

– gas cleaning is aimed only at collecting dust, there is no collection of gaseous substances.

The closest in terms of technical solution and achieved result is the technical solution published in the article “Solving the problems of purifying gas emissions in alumina production” (magazine Non-Ferrous Metals No. 1, 2020, Shepelev I.I. et al., pp. 111-115), which is accepted as a prototype.

According to the prototype, the gas cleaning system has a final stage of flue gas purification - “wet” cleaning in a scrubber, which is irrigated with recycled sludge water containing a caustic alkali of 7.6 g/l. “Wet” gas cleaning allows you to simultaneously capture solid particles and gaseous components. The sub-sludge water is supplied to the scrubber, towards the flue gases, after contact it is removed from the apparatus and sent to the sludge field, while in the scrubber the carbonization of the caustic soda of the sub-sludge water with CO ₂ contained in the flue gases occurs.

The disadvantages of the prototype are:

– one-time use of sludge water in a scrubber (water after one contact with flue gases is directed to the sludge field);

– to irrigate the scrubber, recycled sludge water is used, which has a low content of caustic alkali (7.6 g/l), which does not allow the full absorption of CO ₂ in the flue gases;

– incomplete absorption of caustic alkali in the used slurry water (in the subsludge water after use the caustic alkali content is 6.5 g/l).

Disclosure of the invention

The technical result achieved by implementing the claimed invention is to increase the degree of purification of the exhaust flue gases of calcination furnaces from dust and reduce the level of CO ₂ in the exhaust flue gases due to carbonization of the calcination filtrate.

The technical result is achieved due to the fact that in the method for purifying flue gases from a calcination furnace in the production of alumina, including (a) supplying flue gases from the calcination furnace sequentially to a multicyclone and an electric precipitator to purify said gases from dust, (b) supplying purified gases for wet cleaning into the scrubber, (c) irrigation of gases in the scrubber with a calcination filtrate containing caustic soda with a concentration of at least 16 g/l to obtain a pulp, (d) separation of the pulp into liquid and solid phases, with the liquid phase being re-fed to the irrigation stage ( c), and the solid phase is returned to the alumina production process at the charge sintering stage.

Irrigation stage (c) in the scrubber is carried out until the caustic soda is completely carbonized from the calcination filtrate.

The schematic diagram of the proposed method is presented in the figure, where: 1 – calcination furnace; 2 – multicyclone; 3 – electric precipitator; 4 – collector; 5 – smoke exhauster; 6 – pump; 7 – scrubber; 8 – thickener.

The method is implemented as follows.

Flue gases from the calcination furnace 1 enter the multicyclones 2 and electrostatic precipitators 3, then are sent from the common collector 4 of the calcination furnace 1 through smoke exhausters 5 for subsequent wet cleaning in the scrubber 7. The flue gases enter the lower part of the scrubber 7, and are supplied to the upper part of the scrubber 7 calcination filtrate containing caustic alkali for irrigating incoming gases. The calcination filtrate is one of the finished products after the aluminum hydroxide filtration step and is an alkaline solution. The alkali in the filtrate is caustic, its content reaches from 16 to 20 g/l.

As a result, in scrubber 7, the flue gases are purified from fine dust fractions and carbon dioxide is combined with the caustic alkali contained in the calcination filtrate according to the formula:

2NaOH +CO ₂ = Na ₂ CO ₃ + H ₂ O (1),

Moreover, the supply of a fresh solution of calcination filtrate occurs periodically, since the filtrate is reused (due to the circulation of scrubber - thickener - scrubber) until the caustic alkali is completely converted into soda, with the absorption of CO ₂ .

The pulp obtained after irrigation in the flue gas scrubber 7 with the calcination filtrate is sent to the thickener 8 to separate the liquid and solid phases. The liquid phase (calcination filtrate cleared of solid particles) is re-fed by pumps 6 to irrigate the flue gases into the scrubber 7, and the solid phase (containing aluminum compounds) is returned to the alumina production process at the charge sintering stage.

When the percentage of soda (the ratio of the amount of Na ₂ O associated with CO ₂ to the total amount of Na ₂ O _total ) in the used calcination filtrate reaches a level from 90 wt% to 100 wt%, this solution is periodically pumped out to the soda separation section, and the system replenish with fresh calcification filtrate. Replenishing the system with fresh calcination filtrate will also be necessary as the process of evaporation of the solution in the scrubber proceeds and the entrainment of the resulting water vapor along with the exhaust flue gases.

Carrying out the invention

The implementation of the claimed invention is confirmed by the following example:

The rotary calcination kiln has a capacity of 11.2 tons to 11.5 tons of alumina per hour. Natural gas is supplied for combustion in an amount from 1380 to 1400 m ³ /h. The exhaust flue gases formed after combustion, after passing through the refrigerator and gas cleaning system (multi-cyclones and electrostatic precipitator), enter the scrubber for “wet” cleaning. The flue gases after the electric precipitator have the following parameters: temperature – 145÷155°C, volume – 39.0÷40.0 thousand m ³ /h, CO ₂ content is 7.8÷8.0% vol.

The scrubber was irrigated with calcination filtrate in a volume of 200 ^m3 , with a caustic alkali concentration of 16 to 20 g/l. The pulp obtained after irrigating the flue gases with calcination filtrate was sent to a thickener to separate the liquid and solid phases. The liquid phase was re-fed to irrigate the flue gases into the scrubber, and the solid phase was fed into the process of preparing the sintering charge. Upon achieving complete carbonization of the caustic alkali, in approximately 168 hours, the calcination filtrate was sent to the soda separation unit, and a “fresh” solution of the calcification filtrate was supplied to the scrubber for irrigation. The table presents the results of implementing the method, namely the results of capturing CO ₂ in the flue gases of calcination furnaces. According to the presented data, the process of binding CO ₂ with caustic alkali during solution circulation is visible, which proceeds continuously and ends when the content of caustic alkali reaches zero. The process of CO ₂ capture is confirmed by the indicators of the device (gas analyzer) and the data of laboratory analyzes of the calcification filtrate.

In comparison with the prototype, the use of calcination filtrate is more economical for capture.

We take the volume of sludge water (prototype) and the volume of calcification filtrate equal to 1 m ³ .

Prototype (sludge water):

The amount of CO ₂ captured by subsludge water (according to the article “Solving the problems of purifying gas emissions in alumina production” (magazine Non-Ferrous Metals No. 1, 2020, Shepelev I.I. et al., pp. 111-115), table No. 3):

Amount of CO ₂ = ((18.0 kg/m ³ - 6.5 kg/m ³ ) – (14.92 kg/m ³ – 7.6 kg/m ³ ) / 1 m ³ = 4.2 kg,

where 18.0 kg/m ³ is the _total content of Na ₂ O in the sub-sludge water after the Wet Treatment Unit (WTU); 6.5 kg/m ³ – Na ₂ Ok content in subsludge water after ULV;

14.92 kg/m ³ – total Na ₂ O _content in the sub-sludge water up to the ULV; 7.6 kg/m ³ – Na ₂ Ok content in subsludge water up to ULV.

Calcification filtrate:

Amount of captured CO ₂ :

Amount of CO ₂ = ((20.5 kg/m ³ – 0 kg/m ³ ) – (20.5 kg/m ³ – 17.5 kg/m ³ ) / 1 m ³ = 17.5 kg,

where 20.5 is the content of Na ₂ O _total in the filtrate after circulation; 0.0 kg/m ³ – content of Na ₂ Ok in the filtrate after circulation;

20.5 kg/m ³ – total Na ₂ O _content in the filtrate before circulation; 17.5 kg/m ³ – Na ₂ Ok content in the filtrate before circulation.

From the presented calculation it is clear that to achieve the amount of CO ₂ captured by one cubic meter of calcification filtrate, 17.5 / 4.2 = 4.1 cubic meters of sludge water will be needed, with the same flue gas parameters.

In addition, based on the data obtained in the table, the value of CO ₂ capture during continuous circulation of the calcination filtrate is on average from 4.5 to 5.7% wt. (average capture value based on the results of three examples of different concentrations).

In the prototype, the calculation of CO ₂ capture by sludge water is presented below.

Mass of captured CO ₂ = 50 m ³ / h * 4.2 kg / 1 m ³ = 210 kg / h,

where 50 m ³ /h is the amount of sub-sludge water supplied for one-time irrigation of the ULV; 4.2 kg – the amount of CO ₂ captured from 1 m of sludge water.

The mass of CO ₂ in the exhaust flue gases of one sintering furnace is on average 8000-8200 kg/h - data based on the results of the work used at the sintering furnace plant.

The capture of CO ₂ by sub-sludge water will be = 210 / 8200 * 100 = 2.6% wt.

Based on the above, the claimed CO ₂ capture method is more effective than the prototype, both in terms of the degree of CO ₂ capture and in the more efficient use of the absorbing substance (calcination filtrate).

Claims (2)

1. A method for purifying flue gases from a calcination furnace in the production of alumina, including (a) supplying flue gases from the calcination furnace sequentially to a multicyclone and an electric precipitator to purify said gases from dust, (b) supplying purified gases for wet cleaning to a scrubber, (c) irrigation gases in a scrubber with a calcination filtrate containing caustic soda with a concentration of at least 16 g/l, to obtain a pulp, (d) separation of the pulp into liquid and solid phases, while the liquid phase is re-fed to the irrigation stage (c), and the solid phase is returned into the alumina production process at the charge sintering stage. 2. The method according to claim 1, characterized in that the irrigation stage (c) in the scrubber is carried out until the caustic soda is completely carbonized from the calcination filtrate.