1. Overview of secondary aluminum ash disposal
The aluminum ash disposal project adopts a wet process to hydrolyze, desalinate and desulfate the aluminum ash, and desalt fluoride. The aluminum ash sorted by the project is mixed with a certain amount of quicklime and water in a certain proportion into the pre-reaction kettle, and then enters the hydrolysis kettle for reaction, which removes aluminum nitride and aluminum carbide at a certain temperature, and forms a stable calcium fluoride precipitate and fixes fluoride through the reaction of the leached toxic substance. In addition, every 20 batches, the mother liquor is extracted by evaporation, concentration and crystallization, and the salt in the filter press mother liquor is removed regularly.
The values of aluminum ash content are shown in Table 21。
Table 21List of comprehensive values of aluminum ash content in EIA
2. Description of the secondary aluminum ash disposal process
1) Pre-reaction.
First, the metered water (including tap water and filter mother liquor) is added to the pre-reactor, and then the aluminum ash in the metered aluminum ash bin and quicklime in the quicklime bin are added through the screw conveyor, and the water: aluminum ash: quicklime = 10:5:1 is controlled, and water is sprayed into the pipeline with powder at the same time to reduce the amount of dust in the pre-reaction kettle. Stirring (45 rpm) and mixing are initiated, in which a very small part of the aluminum nitride reacts with water to form aluminum hydroxide and ammonia. In order to reduce the ammonia produced in the pre-reaction stage as much as possible, the feeding process and the mixing process need to be completed quickly, and the pre-reaction time of this project is 05h。After the slurry is mixed evenly, it is immediately transferred to the hydrolysis reactor, after the change, 1 pre-reactor corresponds to 5 hydrolysis reactors, that is, the mixed material of 1 pre-reactor is first added to 1 hydrolysis reactor, and the pre-reactor is then pre-reacted and mixed, and the mixed material is added to 2 hydrolysis reactors, and the operation is carried out sequentially. The main reaction equation is shown in the hydrolysis process analysis.
The quicklime is transported by tanker to the quicklime bin in the plant for unloading, and the dust (G2-1) on the top of the quicklime bin generated during the feeding and discharging process is treated by the silo top dust collector and discharged together with the treated aluminum ash feeding dust.
After a small amount of pre-reaction waste gas (G2-2) generated in the pre-reaction process is collected under negative pressure, it is treated with "water spray (feeding) + water spray", and the treated waste gas is discharged through a 20m high exhaust cylinder (2).
2) Hydrolysis. First of all, the slurry mixed in the previous process is added to the hydrolysis kettle, and then the metered reclaimed water (filter press mother liquor, waste gas treatment facility drainage) is added, and the control water: aluminum ash = 1:1. Under the condition of stirring, the temperature of the kettle liquid is maintained at about 80 by means of steam heating and circulating water cooling, the aluminum nitride and water in the slurry react to produce aluminum hydroxide and ammonia, and the aluminum carbide and water react to produce aluminum hydroxide and methane, and the reaction is an exothermic reaction, and at the same time, the toxic substance fluoride leached in the slurry reacts with calcium hydroxide to form calcium fluoride precipitate.
In the hydrolysis reaction kettle, sodium fluoride, potassium fluoride reacts with excess calcium hydroxide solution to produce stable calcium fluoride, which enters the dry material and finally enters the wallboard; According to the "Hazardous Waste Identification Standard Leaching Toxicity Identification", the concentration limit of hazardous components in inorganic fluoride leaching solution does not include calcium fluoride. Therefore, the leaching toxicity of fluoride is effectively removed.
At room temperature, aluminum ash, quicklime, and water basically do not react, so a heating process is required. The hydrolysis reaction consists of two stages: a heating section and a reaction stage.
Heating section. The slurry in the hydrolysis reactor is heated at room temperature by jacketed steam and controlled by the temperature transmitter of the reactor, so that the slurry is heated to 80 and the steam is stopped. (The vapor pressure is maintained at 0 throughout.)1MPa, temperature 110). The heating time is about 1h. The reactor in the heating section keeps the stirring operation throughout the whole process, and the stirring rate is 45rpm.
Hydrolysis reaction section.
The hydrolysis reaction time is 3-4h, and the stirring speed is 30-45rpm at the same time during the reaction. During the reaction process, samples were taken at intervals of 30min to detect the nitrogen content of aluminum ash in the slurry until the nitrogen content was less than 03%, the reaction solution is transferred to the filter press. The hydrolysis waste gas (G2-3) produced by hydrolysis is collected at negative pressure and treated by ammonia water absorption device (ammonium sulfate reaction device for standby), and then discharged through a 20m high exhaust cylinder (3). Nitride (In terms of n elements, %, %.) conversion rate%, sodium fluoride and potassium fluoride were designed to convert 1
At the beginning of the hydrolysis reaction, steam is required to raise the temperature in the kettle to about 80 degrees Celsius to achieve the optimal temperature required for the hydrolysis reaction. The reaction is an exothermic reaction, a large amount of heat is released during the reaction, and the reaction kettle needs to be cooled in the middle and late stages of the hydrolysis reaction, and the temperature is controlled at 80 90 by using jacketed room temperature circulating water. A temperature transmitter in the kettle is used to control the flow of steam or circulating water.
Main reaction:
aln+3h2o = al(oh)3↓+nh3↑
al4c3+12h2o = 4al(oh)3↓+3ch4↑
Side reactions:
na2o+ h2o = 2naoh
k2o+ h2o = 2koh
cao+ h2o = ca(oh)2
2al+6h2o =2 al(oh)3↓+3h2↑
2naoh+al2o3 = 2naalo2 + h2o
2al+2naoh+2h2o = 2naalo2 + 3h2↑
The leached fluoride (mainly sodium fluoride and potassium fluoride) reacts with Ca2+ in solution as follows
2f-+ ca2+= caf2↓
3) Filter press. The hydrolyzed slurry enters the filter press for filtration, the obtained solid phase (filter cake) is transferred to the drying process, and the liquid phase (filter press mother liquor) is returned to the hydrolysis process. The exhaust gas (G2-4) generated by the filter press process and the exhaust gas (G2-5) of the mother liquor tank generated by the filter press mother liquor tank enter the ammonia water absorption device (ammonium sulfate reaction device for standby) for treatment.
4) Evaporation, concentration, crystallization (desalination).
The filter press mother liquor contains soluble chloride salts, etc., which are reused in the pre-reaction process and hydrolysis process, and the salt is extracted once every 20 batches to reduce the salt content in the filter press mother liquor. The mother liquor of the filter press enters the three-effect evaporator, and the steam is used as the heat source to evaporate, concentrate and crystallize to obtain waste salt (S2). The water vapor produced by evaporation (which may contain trace amounts of ammonia) is indirectly condensed by circulating water to form condensate after salt extraction, which is reused in the pre-reaction process and hydrolysis process.
The water vapor produced by this process is condensed and then pressed back to the mother liquor tank for temporary storage (reused in the pre-reaction process and hydrolysis process and pre-reaction process), and the ammonia that may contain it is basically condensed into the condensate water after salt extraction, even if there is ammonia that has not been condensed into the condensate water, it will also enter the closed mother liquor tank with the reuse water pipeline, because the ammonia produced by the mother liquor tank has been accounted for in the exhaust gas of the mother liquor tank, so the ammonia produced by the evaporation, concentration and crystallization process is no longer separately calculated.
5) Drying. The filter cake is transferred to the drying process, and the hot blast stove with natural gas as the heat source is used for drying (the hot flue gas generated by natural gas combustion is directly added to the drying furnace), and the drying temperature is controlled to 200 220.
The drying waste gas (G2-6) generated by the drying process is treated with "cyclone dust removal", and then discharged together with the pre-reaction waste gas (G2-2) after water spraying (feeding).
Figure 21Process flow of comprehensive utilization of aluminum ash
The ammonium sulfate reaction system is the ammonium sulfate reaction system. There are currently two processes:
1. Ammonium sulfate crystallization system: medium and high concentration ammonia, large air volume directly into the ammonium sulfate crystallizer, after crystal centrifugation, directly made into ammonium sulfate finished products, no evaporation is required (not suitable for small air volume projects);
2. Ammonium sulfate preparation system: hydrolysis reaction section, high concentration ammonia, prepared into ammonia, ammonia water is prepared by reaction with ammonia water and sulfuric acid (concentration 35-40%) ammonium sulfate solution, and finally into the evaporator evaporation crystallization centrifugation to make ammonium sulfate finished products.
1. Ammonium sulfate crystallizer process
1) Ammonium sulfate reaction.
98% sulfuric acid and tap water are used to prepare dilute sulfuric acid, which is used as the spray absorption liquid of the sulfuric acid absorption tower, and then the ammonia-containing waste gas (hydrolysis waste gas, filter pressure waste gas, and filter mother tank waste gas) is collected into the ammonium sulfate absorption tower at negative pressure, and ammonia and sulfuric acid react with ammonium sulfate to obtain ammonium sulfate. After absorbing ammonia, the mother liquor and the filter mother liquor enter the overflow tank, and under the condition of stirring, the mother liquor is supplemented with 98% sulfuric acid and tap water to control the acidity of the mother liquor, and then it is reused for ammonium sulfate reaction. Under the condition of agitation, the mother liquor (about 60 ) of the overflow tank overflows into the crystallization tank.
The absorbed exhaust gas is discharged through a 20m high exhaust cylinder (3).
Reaction principle: H2SO4 + 2NH3 = (NH4)2SO4
2) Crystallization. Under the condition of stirring, the mother liquor (about 60) of the overflow tank overflows into the crystallization tank, and the ammonium sulfate crystallizes at room temperature in the crystallization tank to obtain the crystallized ammonium sulfate.
3) Centrifugation. The crystallized ammonium sulfate is transferred to the centrifugal process, centrifuged to obtain a crude ammonium sulfate, and the filter mother liquor is reused in the overflow tank.
4) Drying. The crude ammonia sulfate product is transferred to the drying process, and the hot blast stove with natural gas as the heat source is used for drying, and the drying temperature is controlled to about 120. After drying, the dry material enters the packaging process.
2. Ammonia absorption system.
The ammonia absorption unit was put into operation. The hydrolyzed waste gas is absorbed by ammonia to produce 9% ammonia products. The exhaust gas of the filter press and the exhaust gas of the mother liquor tank of the filter press are absorbed by "sulfuric acid spray" to produce ammonium sulfate products.
1) Two-stage condensation.
The hydrolyzed exhaust gas enters the two-stage fin heat exchanger for heat exchange, and the condensation is condensed to about 20. The first stage heat exchanger uses circulating water (temperature 30-32) for condensation, and the temperature of high-concentration ammonia is reduced from about 90 to about 45.
The second-stage heat exchanger uses refrigerant to cool down (temperature 0-5) and condense, and the temperature of high-concentration ammonia is reduced from about 45 to about 20.
The ammonia obtained by condensation enters the circulation tank of the first falling film absorber. The condensed ammonia gas enters the falling film absorption process.
2) **Falling film absorption.
The condensed ammonia gas enters the falling film absorber. The new water enters from the first falling film absorber to absorb the tail gas with the lowest ammonia content, and the absorbed ammonia concentration is the lowest, that is, to ensure that the ammonia in the gas phase is dissolved in water as much as possible, and about 1% ammonia can be obtained; The solution that absorbs ammonia gas enters the second-stage falling film absorber, and absorbs the ammonia from the first-stage falling film absorber, and about 4% ammonia can be obtained; Then it enters the first-stage falling film absorber and contacts with a higher concentration of ammonia to improve the solubility of ammonia in water, that is, to increase the concentration of ammonia in ammonia water. After absorbing ammonia in the first stage of the falling film absorber, a concentration of 9% ammonia is obtained. The design ammonia absorption efficiency is 90%.
3) Sulfuric acid absorption.
98% sulfuric acid and tap water are used to prepare 30% dilute sulfuric acid, which is used as the spray absorption liquid of sulfuric acid absorption tower (1), and then the negative pressure of the tail gas after falling film absorption is collected into the sulfuric acid spray tower, ammonia and sulfuric acid react with ammonium sulfate to obtain ammonium sulfate, and the tail gas after sulfuric acid absorption is discharged through a 20m high exhaust cylinder (3).
The exhaust gas of the filter press and the exhaust gas of the mother liquor tank of the filter press are treated by the sulfuric acid absorption tower (2), and the tail gas absorbed by the sulfuric acid is discharged together with the hydrolysis waste gas absorbed by the ** falling film.
4) Evaporation, concentration and crystallization.
Two sulfuric acid absorption towers absorb the saturated absorption liquid into the double-effect evaporator, and use steam as the heat source to evaporate, concentrate and crystallize ammonium sulfate to obtain ammonium sulfate. The water vapor produced by evaporation is indirectly condensed by circulating water to form condensate, which is reused in two sulfuric acid absorption towers.
5) Packing. The ammonium sulfate obtained by evaporation, concentration and crystallization enters the packaging machine and is automatically packed in a closed room. The ammonia water absorption system and the ammonium sulfate reaction system share the ammonium sulfate baler.