DK202200048U1 - Optimized semi-dry process for sintering aluminum silicates in alumina production - Google Patents

Abstract

The invention relates to an apparatus for the treatment of aluminium, which apparatus comprises at least one device for mixing (20, 22, 30, 40) aluminum ore with limestone and sodium and/or potassium carbonate to obtain a mixture, and a calcination reactor (60) for production of the calcinet. A circulating fluidized bed reactor (50) is arranged between the mixing device (20, 22, 30, 40) and the calcining reactor (60) and in which the mixture is precalcined.

Description

DK 2022 00048 U1 —1— Optimized semi-dry process for sintering aluminum silicates in alumina production The present invention describes an apparatus for processing aluminum, which apparatus comprises at least one device for mixing aluminum ore with limestone and sodium and/or potassium carbonate to obtain of a mixture, and a calcination reactor for producing the calcine. The production also concerns the associated process.

Production of metals and their salts often depends on thermal treatment of the mined ores, where the naturally occurring metal salts are converted into the corresponding oxides, which, as necessary, are subjected to further processes that yield the target product.

Aluminum oxide (Al20s) is produced predominantly from bauxite in the Bayer process. The mined and milled ore is treated with sodium hydroxide at elevated temperatures of 150 to 200 °C and forms soluble sodium aluminate (NaAlO2), which precipitates as aluminum hydroxide (AI(OH)s) from the resulting supersaturated solution. | the following calcination step forms AlzZOs at temperatures of >1000 °C.

For the Bayer process to be economically viable, the silica content of the ore must be below 10%. Higher silica levels result in simultaneous dissolution of alumina and silica, during which process insoluble sodium aluminum silicates are formed, significantly increasing the amount of sodium hydroxide per tonnes of Al2O3 produced and thus the total yield.

The addition of limestone (CaCOs3) to the raw ore in such a way that the CaO/SiO2 molar ratio is equal to 1 during the sintering procedure, while maintaining the caustic ratio M20/Al2Os (M' = K+ or Nat)?2 , allows the processing of

DK 2022 00048 U1 —D- the aluminum silicates. The typical temperature range in the sintering process of from 1200 to 1350 °C results in the formation of insoluble calcium silicate phases (CaSiO3).

The sintering process is classified as wet, semi-wet or dry sintering, depending on the water content of the feed material. The industrially used wet sintering process uses a slurry-like feed material with an approximate water content of 30% by weight. The semi-dry sintering requires granulation of the feed material with a residual water content of 10-15% by weight. A dry sintering method is possible if the caustic ratio is equal to 2 and if only calcined limestone needs to be added to adjust the CaO/SiO2 ratio accordingly.

The wet sintering ensures agglomerates and therefore easy handling. However, it also suffers from the sticky consistency of its feedstock, resulting in a high energy requirement due to additional vaporization energy. Ovens with long residence times must be used for drying, calcination and subsequent sintering, which results in a high specific energy consumption of 1200-1300 kcal/kg (sinter).

On the other hand, the dry sintering process for sintering nepheline to produce alumina is the most efficient method in terms of energy consumption (650-700 kcal/kg (sinter)). The main advantage of (most of) the dry process is the precalcination of the limestone in a flash precalcinator before sintering in the rotary kiln. This allows for much higher throughputs and shorter rotary kilns, because most of the calcination does not take place in the kiln. However, the dry sintering process is limited to ores with a caustic ratio of 2. An addition of the low-melting carbonates (Na2CQOs: 857 °C, K2CO3: 891 °C) causes material to stick to the furnace wall during the sintering process, making this process impossible, if the caustic ratio needs to be adjusted.

DK 2022 00048 U1 —3— A semi-dry sintering process is described, for example, in CN 105 540 627. In it, the Bayer base red mud powder, lime, bauxite, sodium carbonate and powdered coal are uniformly mixed with the aim of becoming raw material pellets, and the raw material pellets are sintered to form the slags of the alumina produced by the sintering process using two rotary kilns. Such a semi-dry sintering method combines the advantages of wet and dry sintering. This results in smaller size furnaces and thus a lower energy consumption of 800-900 kcal/kg (sinter) compared to the wet sintering method. However, a further reduction of energy consumption, lower residence times and handling of low-melting bases is necessary to further reduce the energy consumption of the process.

Therefore, the underlying purpose of the present invention is to provide a plant for handling aluminosilicate ores with a high silicon content as an economically ecologically feasible process.

A plant with the features according to claim 1 solves this task.

According to the invention, the installation of the plant consists of at least one device for mixing aluminum raw ore with limestone and sodium and/or potassium carbonate. It also includes a calcination reactor for the production of the calcinet. The basic idea behind the production is that between mixing and calcination, a circulating fluid bed reactor is foreseen for precalcining the mixture.

Due to the remarkable heat and mass transfer in a circulating fluidized bed (CFB), it is possible to use bases with low melting points. In addition, customized stay times can be set. Therefore, the total energy consumption can be lowered. This particularly enables the handling of ore with a high silicon content of over 5.5% by weight.

DK 2022 00048 U1 —4— In the mixing step, aluminosilicate raw ore, limestone and recycled solids are mixed with an aqueous M2COs (M* = Na* or K') solution, whereby the caustic ratio is adjusted to 2, at the same time as sufficient moisture is added to bind all the components for the subsequent granulation. It is also possible to combine the mixing and granulating step in at least one granulator. It is also possible that the mixing takes place during drying or preheating. An energy-intensive drying step for the respective carbonates is unnecessary, since they are supplied as a salt solution.

In a preferred embodiment, the resulting mixture is granulated in two series-connected granulators. The formation of granules encapsulates the Na2CO3/K2COs3 additive in the grains.

These small pellets greatly facilitate the establishment of a circulating fluidized bed. In addition, dust generation is less if the material is granulated. This was shown in pilot-scale sintering tests comparing semi-dry and dry processes. Furthermore, granulation does not only include a reduction of the water content, but also a reintegration of dust from downstream stages, which makes the process more resource efficient.

The Na2CO3/K2CO3 solution added in the granulation process not only supplies the required bases to the solid mixture, but also acts as a binder and decisively improves the granulation process. This was shown in a series of laboratory granulation tests.

The grain size is typically between 5-20 mm, which is most preferred for fluid bed systems.

DK 2022 00048 U1 —5- It is particularly preferred to use at least two granulators in order to carefully control the final moisture content (preferably 10-13% water content by weight).

With respect to the final calcination step, the type of reactor used may be a furnace. This has the advantage that rotary kilns are a relatively cheap and very well-known technology. However, it is also possible to use any type of fluid bed reactor to improve the heat and mass transfer.

In a preferred embodiment, a ratio R of (Na 2 O+Kz 2 O)/Al 2 O s is 0.95<R<1.05 to ensure a high product quality. Therefore, a control unit is used to control or regulate the quantities of ore and base that are mixed in the granulation.

Furthermore, it is particularly preferred that the sodium and/or potassium carbonate is a salt solution that is stored in granulation liquid tanks. It is possible to add water to control the concentration of the solution. Therefore, it is also possible to work with an under- or over-saturated solution. Further energy savings can be achieved by supplying the granulation tanks with an (oversaturated) salt solution from the refinery's salt plant. This saves additional evaporation energy in the salt plant, which corresponds to approx. 30 kcal per kg of sinter produced (depending on the concentration of the salt solution).

In another embodiment, at least one, preferably 2 to 3 pre-heating steps are provided for the mixture and/or the granules. The material is typically heated from ambient temperature to approx. 200 °C. This makes it possible to further improve energy efficiency, especially with the help of any heat recovery concept.

DK 2022 00048 U1 —6— Additionally or alternatively, a drier is provided for the mixture or the granules before the circulating fluid bed reactor.

It is therefore possible to reduce the water content, preferably to a value of less than 1% by weight, and consequently also the energy consumption.

The granules have a very good drying behavior, which keeps the rotary dryer relatively short.

The obtained mixture is preferably precalcined in the circulating fluidized bed reactor at 700 to 900°C, preferably at 800 to 850°C to achieve a sufficient precalcination rate.

The final calcination step in the calcination reactor typically takes place at 800 to 1400 °C, preferably at 1000 to 1300 °C.

The average residence time in the circulating fluid bed reactor is between 15 and 25 minutes, preferably 20 +/- 2 minutes, and/or the average residence time in the calcination reactor is between 30 and 200 minutes.

These residence times, especially if they are correlated, lower operating temperatures and avoid liquid base fractions.

With a view to a further reduction of the plant's energy requirements, a first recovery line has been provided for the recirculation of hot exhaust gases from the calcination reactor in the circulating fluid bed reactor and/or in at least one preheating stage.

Alternatively or additionally, a second recovery line is provided for recirculation of hot exhaust gases from the circulating fluidized bed reactor in at least one preheating stage.

In order to achieve an even higher degree of optimization of the overall energy balance, at least one cooler has been provided to cool the calcine formed in the calcination reactor.

Preferably, a third recovery line is provided for recirculation of hot exhaust gases from the cooler in the calcination reactor and/or in the circulating fluid bed reactor and/or in at least one preheating stage.

DK 2022 00048 U1 —7-— In addition, a channel supplies a gas with an oxygen content of between 15 and 25 percent by weight as fluidization gas into the circulating fluid bed reactor. Thereby, the fluidizing gas is simultaneously used as an Oz source for chemical reactions. The use of air or oxygen-enriched air as a cheap oxygen source is preferred. With regard to the circulating fluid bed, it is also preferred that the circulating fluid bed reactor is configured such that at least 70 percent by weight of the carbon contained in the ore is removed. This results in a reduction of the total mass flow in the calcination step. Furthermore, the carbon combustion provides at least parts of the energy required for the endothermic calcination, which is optimally transported within the circulating fluidized bed. Therefore, the purpose of the production is solved by the measures presented, so that the handling of aluminosilicate ore with a high silicon content based on a semi-dry sintering process is possible. The combination of the known advantages of the semi-dry basic process across the established methods is achieved with reasonable energy consumption.

Furthermore, a process is described herein. . Such a process includes the steps of (a) mixing aluminum ore with limestone and sodium and/or potassium carbonate to form a mixture, and (b) calcining the mixture to calcine. As the essential step, the mixture between step (a) and step (b) is precalcined in a circulating fluidized bed.

Preferably, the moisture content of the mixture or the granules introduced in the precalcination is 10 to 15% by weight, which gives the granules a good stability.

DK 2022 00048 U1 —8- In addition, in a preferred embodiment, at least 70 percent by weight of the carbon contained in the ore is removed during precalcination with a view to reducing the total mass flow.

Additional features, advantages and possible uses of the invention can be taken from the following description of the figures and the examples of embodiments. All described and/or illustrated features constitute the object of the invention in itself or in any combination, independently of their inclusion in the claims or their reference.

In the figures, it applies that: Fig. 1 shows the plant according to the invention schematically.

Figure 1 shows the main structure of a plant according to the invention. Via channel 10, aluminum oxide ore is fed into a homogenization silo 11, where it is mixed with recycled solids and/or limestone from feed via channels 13 and 14 to form a mixture.

Channel 15 supplies at least one granulation liquid tank with a salt solution consisting of sodium and potassium carbonates, which can be diluted if necessary with water injected via channel 16.

The mixture as well as the compound salt solution is then dosed via channels 17 and 18 to a first granulator 17, where it performs three tasks: adding the required amount of bases to the solid mixture of limestone, nepheline and dust (and possibly some recycled materials from the process), providing moisture for granulation and functioning as a binder. | in this embodiment, the granulator 20 also functions as a mixing device. However, it is also possible to foresee an addition mixer before the granulator 20. It is highly advisable, but

DK 2022 00048 U1 —9— not necessary to have another granulator 22, preferably connected in series via channel 21, in order to achieve the necessary granulation time, but also to have a good control over the final humidity. In this connection, it is possible to add additional water via channel 24.

Downstream of the granulators 20, 22, the produced granules are supplied via channel 23 to a dryer 30. There, the wet granules are dried, often with hot flue gases from channel 63. The dryer 30 is preferably designed as a rotary dryer.

The waste gases are led via line 32 into an electrostatic filter 33 and from there via channel 35 into a waste gas treatment (not shown). Via channel 34, small particles that are filtered out in the electrostatic filter 33 are fed into channel 45 for recycling.

The dried granules are fed via channel 31 into a preheating section 40. It is preferred that the preheating section 40 comprises 2 or 3 stages. Furthermore, the following venture/cyclone stages lead to particularly good results. With a view to an improved energy concept, hot gases can be used that are supplied via channel 53, preferably upstream.

Via channel 42, the waste gases from the preheating section 40 are fed into an electrostatic filter 43. From there, parts of the waste gases are preferably extracted as exhaust gas via channels 46 and 48, while the other part is fed via channels 46 and 47 as a gas for carbonization. Filtered particles can be fed via channels 44 and 45 into the first granulator 20.

After preheating, the granules are fed via channel 41 into a circulating fluidized bed reactor 50 for precalcination, which is in accordance with the production to ensure the necessary very good heat and mass transfer. There, the granules are decarbonised to a degree of at least 80% by weight. The precalcined particles are fed via channel 51 into a calcination reactor 60. Via channel 78 and/or 52

DK 2022 00048 U1 —10— fluidizing gas is injected over the bottom nozzle grid.

It is preferred that approx. 20 volume percent of the fluidizing gas is fresh air, whereby this percentage of fresh air can differ, depending on the design of the circulating fluid bed.

Preferably, the largest part of the fluidizing gas is air from a downstream device, which is transferred to the precalcination reactor via channels 76, 78, a so-called tertiary air channel.

Furthermore, it is possible to lead hot waste gases from the circulating fluidized bed with its large amount of waste gases into the preheating stage 40 via channel 53. There it can be used as a direct and/or indirect heat transfer medium.

There is also no need for separate handling of waste gas.

The final sintering takes place in the calcination reactor 60, which is preferably designed as a rotary kiln in order to reduce the amount of hot gases compared to fluid bed technology.

Fresh air is introduced into the calciner reactor via line 62, while it is also possible to introduce hot gas from the downstream cooler via channels 76 and 77 to reduce the amount of energy to be supplied.

The resulting hot sinter is fed via channel 61 into a cooler 70, where it is preferably air-cooled by a grate cooler.

The cooler 70 is preferably cooled with air introduced via channel 73. This air is extracted and used at least partially as a heat transfer medium in the precalcination reactor 50 and the calcination reactor 60. Furthermore, air can be extracted via line 74 into an electrostatic filter 75 and from there via channel 75 into a non-shown waste gas treatment.

Cooled product is extracted via channel 71, 72, whereby it is possible to mix in small particles filtered out in the electrostatic filter 75.

DK 2022 00048 U1 —-11-— Reference numbers channel

11 homogenization silo 12 granulation liquid tank 13-18 channel 20 granulator

10 21 channel 22 granulator 23, 24 channel 30 dryer 31, 32 channel

33 electrostatic filter 34, 35 channel 40 preheating section 41, 42 channel 43 electrostatic filter

44-48 channel 50 circulating fluidized bed reactor 51 channel 60 calcination reactor 61, 62 channel

70 cooler 71-74 channel 75 electrostatic filter 76, 77 channel

Claims (12)

DK 2022 00048 U1 —-12- Utility model requirements 1. Apparatus for processing aluminium, which apparatus comprises at least one device for mixing (20, 22, 30, 40) aluminum ore with limestone and sodium and/or potassium carbonate to form a mixture, and a calcine ring reactor (60) for producing the calcinet, in which a circulating fluidized bed reactor (50) is placed between the mixing device (20, 22, 30, 40) and the calcining reactor (60), and in which the mixture is precalcined, which is new by , that a first recovery line (63) is provided for recirculation of hot exhaust gases from the calcination reactor (60) in the circulating fluid bed reactor (50) and/or in at least one preheating stage (40) and/or the dryer ( 30), in that a second recovery line (53) for recirculation of hot exhaust gases is provided from the circulating fluidized bed reactor (50) in at least one preheating stage (40) and/or the dryer (30), in that there is provided a cooler (70) for cooling the calcine that is formed in approx the calcination reactor (60), and in that a third recovery line (76, 77, 78) is provided for recirculation of hot exhaust gases from the cooler (70) in the calcination reactor (60) and/or in the circulating fluid bed reactor (50) and/or in at least one preheating stage (40) and/or a dryer (30), where the at least one preheating stage (40) and/or a dryer (30) and/or a granulator (20/22 ) is used as the mixing device. 2. Apparatus according to claim 1, which is novel in that at least one granulator (20, 22) is provided to form granules of the mixture fed into the circulating fluid bed reactor (50). 3. Apparatus according to claim 1 or 2, which is new in that at least two granulators (20, 22) form granules of the mixture which are fed into the circulating fluid bed reactor (50). DK 2022 00048 U1 —13- Apparatus according to any one of the preceding claims, novel in that the calcination reactor (60) is a rotary kiln. 5. Apparatus according to any one of the preceding claims, which is novel in that a control unit ensures that the ratio R between (Na2O+K2O)/Al2O3 is 0.95<R<1.05. 6. Apparatus according to any one of the preceding claims, which is novel in that the sodium and/or potassium carbonate is a salt solution stored in granulation liquid tanks (12). 7. Apparatus according to any one of the preceding claims, which is novel in that at least one pre-heating step (40) is provided for the mixture and/or the granules. 8. Apparatus according to any one of the preceding claims, which is novel in that a dryer (30) is provided for the mixture or granules before the circulating fluidized bed reactor (50). 9. Apparatus according to any one of the preceding claims, which is novel in that the circulating fluid bed reactor (50) is designed for an operating temperature of between 700 and 900 °C. 10. Apparatus according to any one of the preceding claims, which is new in that the circulating fluid bed reactor (50) is designed for an average residence time of between 15 and 25 minutes, and/or the calcination reactor ( 60) is designed for an average residence time of between 30 and 200 minutes. DK 2022 00048 U1 — 14 — 11. Apparatus according to any one of the preceding claims, which is new in that a channel (52) supplies a gas with an oxygen content of between 15 and 25% by weight as fluidization gas into the circulating fluid bed reactor (50 ). 12. Apparatus according to any one of the preceding claims, which is novel in that the circulating fluid bed reactor (50) is configured so that at least 70% by weight of the carbon contained in the ore is removed.