JPH0610084B2 - Method for removing carbon compounds in the circulating fluid of the buyer process - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a process for producing an alumina hydrate or alumina by the Bayer method, in which a carbon content in an organic compound and a carbon compound such as sodium carbonate is removed from the process material. Regarding how to remove.

More specifically, the carbon compound-containing Bayer process circulating liquid in which the molar ratio of the A component to the Na component is adjusted to a range of A ₂ O ₃ / Na ₂ O = 1 to 5 and / or a substance capable of concentrating and solidifying the circulating liquid, The present invention relates to a method of removing a carbon content in an organic compound and a carbon compound such as sodium carbonate by performing heat treatment in a temperature range of 500 to 1350 ° C.

[Prior Art] To produce an alumina hydrate or alumina by the Bayer method, an alumina-containing ore, usually bauxite ore, is digested with an aqueous solution of caustic soda at high temperature and high pressure, and the alumina content in the ore is dissolved and extracted as sodium aluminate. That, seeds (alumina hydrate crystals) are added to the sodium aluminate clarified liquid after separating insoluble matter such as red mud and stirred to cause a hydrolysis reaction to precipitate alumina hydrate crystals, The obtained alumina hydrate crystals are separated and washed, dried to obtain a product as it is, and further subjected to high temperature firing to obtain an alumina product. Then, the mother liquor from which the alumina hydrate crystals have been separated and a part of the washing liquid are concentrated and reused in the process again.

As is clear from the above description, in the Bayer method alumina production process, the liquid containing the Na component and the A component is continuously circulated in the process. In the present invention, all of these liquids are referred to as the buyer process circulating liquid.

In the present specification, the carbon compound means all carbon compounds including organic compounds such as sodium oxalate and sodium carbonate.

The carbon compound enters the circulating liquid of the buyer process because the organic substances (humin, bitumen, butene, etc.) in the raw ore dissolve and accumulate in the digestion process, and the synthetic or natural coagulating sedimentation agent added during the process or organic matter. The main causes are accumulation of defoaming agents, carbonates in the raw caustic soda, and accumulation of sodium carbonate due to carbonation due to contact of the process circulating liquid with air.

When a large amount of carbon compounds are present in the circulating liquid of the Buyer process, the precipitated alumina hydrate becomes finer crystals and the precipitation rate decreases, operational troubles due to scaling in the Buyer process equipment, solid-liquid alumina hydrate crystals It is known that the separability is deteriorated, the whiteness of the product alumina hydrate is deteriorated, and the productivity of the buyer process and the product quality are deteriorated.

Therefore, for these reasons, there is a strong demand for the development of an efficient method for removing carbon compounds in the circulating liquid of the buyer process.

Several proposals have hitherto been made as methods for removing carbon compounds in the circulating liquid of the buyer process. That is, a pressurized oxygen method (Japanese Patent Publication No. 45-30548), an ultraviolet irradiation method (Japanese Patent Laid-Open No. 49-20097), a cooling method (US Pat. No. 3,508,884), a magnesium hydroxide coprecipitation method (Japanese Patent Laid-Open No. 51-130692). Causticizing method (US Pat. Nos. 3120996 and 3341286), sodium oxalate removal method (US Pat. Nos. 3649185, 33)
No. 72985, Japanese Patent Publication 48-11480, Japanese Patent Publication 53-
398 and Japanese Patent Publication No. 53-400), bauxite firing method (JP-A-47-21395) and the like. Of these, the main ones industrially put to practical use are the causticizing method and the sodium oxalate removing method.

The causticizing method separates lime carbonate produced by reacting sodium carbonate in the buyer's circulating liquid with slaked lime, and the treatment liquid is not only low in concentration but also insufficient in removal rate of carbonate,
Substantially only the carbonate can be removed.

Sodium oxalate removal method, since sodium oxalate also crystallizes as fine particles in the alumina hydrate precipitation step, the sodium oxalate is dissolved by filtration and washing treatment of the alumina hydrate, and hydrated lime is added to this solution to add oxalic acid. This is removed as calcium. However, this method has the drawbacks that the crystallization conditions of sodium oxalate are limited and that water is newly brought into the process for the dissolution of the sodium oxalate crystals.

The present inventors have developed a method which is superior in treatment effect as compared with the conventional methods as described above, does not contaminate the circulating fluid due to substances outside the process, and can be removed regardless of the type of carbon compound (Japanese Patent Publication No. 58-46450).

To summarize the method, the molar ratio of the A component to the Na component of the Bayer process circulating liquid containing the A component, the Na component, and the carbon compound and / or the substance for concentrating and solidifying the Bayer process circulating liquid is A ₂ O ₃ / Na ₂ O = 1-5, and then heat treatment in the temperature range of 500-1350 ° C. to substantially decompose sodium aluminate and carbon dioxide to remove carbon compounds as carbon dioxide. Is the way.

It is desirable to make this molar ratio larger than 1 in order to sufficiently suppress the residue of sodium carbonate, but if this ratio becomes too large, the heat load during the heat treatment increases and it is disadvantageous in terms of energy economy. It is desirable that this ratio be within 5. A more preferred range of A ₂ O ₃ / Na ₂ O molar ratio is 1-2.

The temperature range of the heat treatment is 500 to 1350 ° C, more preferably 700 to 1000 ° C. 400-500
At ° C, only the sodium content bound to the intermediate carbon compound is converted to sodium carbonate, and the effect of removing the carbon compound cannot be expected. Further, at a temperature above 1350 ° C., the heated product is melted to make the operation extremely difficult, and it is disadvantageous in terms of energy economy.

The inventors of the present invention have since then improved the process and equipment in the industrial implementation stage of the above method.

[Problems to be Solved by the Invention] The heat treatment of the concentrated solidified product of the buyer process circulating liquid is
For example, it is a method of heating in a heating device such as a rotary kiln to thermally decompose the carbon compound contained in the concentrated solidified product and removing it as carbon dioxide gas.

At this time, the sensible heat of the combustion exhaust gas such as a rotary kiln at 500 to 800 ° C. was used for evaporation, concentration, drying and heating of the Bayer process circulating liquid and / or the concentrated liquid. (Note that
For the evaporation, concentration, drying, and heating of the concentrated liquid, hot air from a separately provided hot air generator may be used instead of combustion exhaust gas such as a rotary kiln. At that time, the exhaust gas from the kiln will be recovered by the other device to recover the effective energy. ) During this evaporation / concentration / drying / heating process, the concentrated liquid is granulated. Therefore, this evaporation / concentration /
The granulation / drying / heating step is accomplished by using a concentrated liquid as a raw material and a kiln combustion exhaust gas as a heat source, for example, by a granule forming apparatus as shown in JP-B-63-35572. .

The granules may be directly supplied to the rotary kiln and heat-treated, but in order to facilitate the treatment in the rotary kiln, the present inventors classify the granulated granules with a wind classifier (such as a cyclone) to obtain large particles. A method was developed in which the granules were heat treated and the remaining granules were returned to the granulation process and circulated.

In this method, the kiln combustion exhaust gas or hot air passes through these devices and the temperature drops to 100 to 300 ° C., but a few 10 g of fine dust that cannot be collected by the cyclone.
/ NM ^{3 is} included. This uncollected floating powder cannot be scattered into the atmosphere from the viewpoint of preventing air pollution.

Therefore, although the uncollected fine dust is collected by a dry dust collector (bag filter, electric dust collector, etc.) or a wet scrubber, this collection still has the following problems.

The dust collected by the electric precipitator is 5μ.
It was a fine dust of about m, and when returned directly to the granule molding machine, the particle size of the powder at the outlet of the granule molding machine became finer than before collecting. When the operation is continued in this state, the circulation amount of fine dry powder increases, and the load of the wind classification device increases,
Along with that, it causes an increase in piping pressure loss.

If unavoidably the granules having such a small particle size are supplied to the kiln, dusting is caused in the kiln, an incomplete combustion state occurs, and it becomes substantially difficult to maintain the operation. In order to continue the operation, it is necessary to reduce the combustion air in the kiln (to reduce the wind force) and suppress dusting. That is, it needs to reduce fuel consumption. This leads to a decrease in the amount of concentrated liquid to be charged, which leads to a large decrease in the efficiency of the apparatus, a deterioration in the basic unit such as electric power, and a decrease in the processing capacity, which is not an economical method. To prevent that,
A large diameter kiln is required, and the amount of capital investment becomes enormous.

Even if the kiln exhaust gas is processed by another system and the hot air for the drying system is used, if the fine dust collected by the electrostatic precipitator is returned directly to the granule molding device, the particle size distribution of the granules becomes smaller. Since it shifts, it is not possible to maintain the target particle size by circulating a large amount of dry powder in the system of the granule forming device, the air classifying device and the dry dust collector, so the above equipment becomes huge and equipment investment It becomes uneconomical in terms of amount.

Therefore, inevitably, instead of an electric dust collector, a water scrubber was used to wash the exhaust gas containing fine dust that could not be collected by the cyclone, and the wash water was circulated to the buyer process to release the exhaust gas to the atmosphere.

However, this fine dust is a particle in which primary particles with a diameter of about 0.5 μm are aggregated, and is a secondary aggregated particle with many voids and with a diameter of 2 to 10 μm. Therefore, it is a particle that collapses in a state of being suspended in the air flow. Further, the ratio of A ₂ O ₃ / Na ₂ O is lower than that in the evaporative concentration step, and the A content and unreacted Na ₂ O (for example, sodium carbonate and sodium hydroxide) and salt contained in the circulating solution of the buyer step are not included. It is a fine powder that contains impurities, is easy to crystallize due to a temperature drop, and absorbs water in the exhaust gas, and is highly deliquescent. Therefore,
When this highly reactive fine powder is treated with a water scrubber, a scale mainly consisting of dawsonite (NaACO ₃ (OH) ₂ ) is crystallized in the exhaust gas duct and the like, and the pressure loss of the scrubber system is increased. In that case, it is unavoidable that the processing capacity gradually decreases as in the case of using the electrostatic precipitator. If it is left as it is and the operation is continued, not only the efficiency gradually decreases, but also it becomes difficult to continue the operation, and it becomes necessary to set the suspension of operation mainly for the scale removing work on a regular basis. In addition, it is necessary to provide a separate scale removing facility, which leads to an increase in capital investment and a decrease in device efficiency.

The present invention eliminates adverse effects on the operation of the granulation process and the heat recovery system of the exhaust gas generated by recovering the fine dust in the exhaust gas from the cyclone, the recovery process of the cyclone, etc., and there is no pollution to the environment. The purpose is to recover as an active ingredient and to improve the processing capacity of this processing apparatus.

[Means for Solving the Problems] The present invention establishes a method for efficiently removing carbon compounds in a circulating liquid of a buyer process, thereby increasing productivity of the buyer process itself, which is difficult to treat conventionally. The purpose is to recover the fine dust and improve the carbon removal capacity.

That is, according to the present invention, the molar ratio of the sodium content and the aluminum content in the treated liquid or the concentrate thereof is A ₂ O ₃ / Na ₂ O = 1 to 1 as a liquid to be treated, which is a circulating liquid of the buyer process in the alumina manufacturing process by the Bayer method. Adjust to the range of 5,
The liquid to be treated and / or its concentrate is granulated, the granules are classified by air, the large granules are heat-treated at 500 to 1350 ° C., the remaining granules are returned to the granulation step and granulated again, The gas after classification is treated with a dry dust collector, the collected fine dust is tumbled and compressed, and the fine dust is returned to the main process. Removal of carbon compounds in the circulating liquid of the buyer process. In law.

Wind power classification is to efficiently and easily perform processing in a heat treatment device such as a kiln, and the particle size and amount of heat treatment with a kiln and returning to the granulation process are a balance of the entire system. It is appropriately determined by.

An example of the present invention will be described below with reference to the flowchart of FIG.

The buyer process liquid from which carbon content should be removed can be processed in principle at any place in the buyer circulation process, but in order to perform efficient treatment, organic compounds before hydrolysis, It is efficient and preferable to treat a high-concentration liquid such as sodium carbonate and sodium oxalate. For example, Japanese Patent Publication No. 63-6490 and Japanese Patent Publication No. 1-14 proposed by the present inventors.
It is one preferred embodiment to use a process solution in which sodium oxalate is concentrated as much as possible according to the method of No. 173.

If necessary, the undissolved residue of this step is added to such a step solution, and the molar ratio of sodium and aluminum in the concentrate is adjusted to A ₂ O ₃ / Na ₂ O = 1 to 5 (however,
This adjustment is the component ratio of the concentrate after concentration, and can be performed after concentration). This concentration is preferably carried out in multiple evaporators. The concentrated process liquid is sent to the granulation process.

In the granulating step, the circulating granules from the dry classification cyclone (1) for air classification and the heated granules collected by the dust collecting cyclone (2) are fed and the concentrated liquid is also added. Are sprayed onto the granules and granulated.

This granule forming device is not limited, but has a rotating paddle in a horizontal cylinder as shown in cross section in FIG. 2 such that the concentrate is sprayed at multiple locations. The form is preferable.

In this case, the granule forming device may not be heated, and may be an external heating system or an internal heating system.

When granulating, the heated circulating granules enter the granule forming equipment (a kneading machine such as a bag mill) and the sprayed concentrated liquid coats the surface of the heated granules. Evaporate, the non-water components of the concentrate deposit on the granules and grow into larger granules. Therefore, the larger the granules put into the granule molding facility, the larger the treated granules.

The granules taken out from the granule forming device are sent to a wind classification process using, for example, a high temperature rotary kiln exhaust gas. In this case, the granules and the exhaust gas exchange heat, and the granules are also heated to a high temperature. Of course, the gas for classifying the wind power is not limited to the exhaust gas from the rotary kiln, and needless to say, may be any heating gas.

Of the granules transported to the cyclone (1) by the exhaust gas, the large granules are collected here, and some are sent to the rotary kiln and some are sent to the granule molding facility. Fine particles that could not be collected by the cyclone (1) are collected by the cyclone (2) and circulated to the granule forming facility.

Granules sent to the rotary kiln are 500-1350
Heat treatment is performed at a temperature of 90 ° C., preferably 900 to 1100 ° C., the carbon compound is decomposed, carbon dioxide gas is released, and the carbon content is removed.

Since the rotary kiln is controlled at a constant heating temperature, fuel oil and combustion air are determined in proportion to the amount of solids charged in the rotary kiln, and the facility capacity is also determined by this.

The decarbonized heat-treated granules are dissolved in the process mother liquor or water and circulated to the buyer process. In addition, the undissolved residue can be prevented from being lost in the sodium content and the aluminum content if it is circulated to the stock solution adjusting step.

Here, since the exhaust gas from the cyclone (2) still contains fine dust containing active ingredients, it is separated by an electric dust collector or a dry dust collector such as a bag filter, and the gas is released into the atmosphere.

On the other hand, the separated fine dust can be used for any purpose as long as it is a device having rolling and compression functions, but if it is a screw conveyor, it can be fed from the electrostatic precipitator to the granule forming device. This is preferable because it can be performed simultaneously. When transported by a screw conveyor, the fine powder is granulated to have a rounded granular diameter of about 20 μm to 5 mm.

Therefore, it may be granulated by an ordinary granulator and fed directly into the kiln, or may be circulated anywhere in the granulation, classification, and concentration steps of this removal step, but from the practical viewpoint, it is a granule compactor device. Is a preferred embodiment.

[Operation] The reason why the process of the present invention is successfully applied is considered to be as follows.

That is, since the fine dust collected by the conventional electrostatic precipitator causes fineness of the agglomerates obtained in the granulation step because it is fine, dusting occurs in the rotary kiln,
This makes it difficult to continue operation, which causes a reduction in the treatment base.

In addition, because of the small size, the short path of the collected dust increases in the granule forming device, the amount of circulating dust increases, and if the circulating dust increases, the pressure loss of the circulating pipe increases. In addition, this fine dust is a dust that easily absorbs moisture, is highly deliquescent, and contains NaC, etc. Since scale accumulates in the circulation pipe, it is necessary to enlarge the equipment such as the circulation pipe and the dust collector. There is no choice but to invest equipment and labor to remove the

For this reason, in the past, the fine dust that could not be collected by a cyclone was treated with a water scrubber to treat the fine dust without returning it to the system, and the entire system was stabilized. Was meant to reduce the removal ability of.

In this case, when the particle shape of the fine dust contained in the exhaust gas before and after being applied to the electrostatic precipitator was examined, particles having a unit particle size of about 2 μm grew into aggregated particles with many voids of about 3 to 8 μm. It was It is presumed that this is due to the fact that the particles are fine and the surface activity of the particles is high, and that electric energy is applied to the surface of the dust collecting electrode, causing the particles to aggregate. However, since the particles having a large number of voids are agglomerated by point contact with each other, the particles have a cohesive force that causes them to collapse once placed in an atmosphere with an air flow. If electrostatically precipitated dust is aggregated with a weak binding force into a coarser and stronger agglomerate, it can be returned to the granule forming equipment, and the carbon removal capability is dramatically improved.

As a result, when the particle layer of the particles collected by the electrostatic precipitator is subjected to rolling and compression motions, the porosity of the particles is reduced, and an appropriate amount of water becomes a binder, which makes it difficult to disintegrate. It is thought that the present invention has been completed because the particles became coarse particles of about 5 mm.

[Example] In order to clarify the effect of the present invention, an experiment was conducted according to the flow shown in and of FIG.

According to the method of the present invention, the circulating liquid of the buyer process is adjusted to a ratio of A ₂ O ₃ / Na ₂ O = 1.2, concentrated to a specific gravity of 1.70, and the concentrated liquid is subjected to cyclone (1 ), And the circulating granules from (2) are fed to the granule compactor (4). The formed granules are sent to the cyclone (1) by the exhaust gas of the rotary kiln (7) to separate large-diameter granules, a part of which is the rotary kiln (7).
The rest is returned to the granule forming device (4) together with the granules separated by the cyclone (2). Here, unseparated fine dust is collected by an electric dust collector (3), rolled by a screw conveyor, compressed, and then supplied to a granule molding apparatus.

The method (2) is a method in which fine dust that has not been separated in the cyclone (2) is collected by the water ejector (3 ') by the buyer process liquid and circulated to the buyer process.

The method of (2) is a method in which the screw conveyor in the method of (3) is simply replaced by a rotary valve (8 '), and a method of returning the fine dust collected by the electrostatic precipitator to the granule forming apparatus as it is.

With the equipment of ~, cyclone, granule forming machine, fan, rotary kiln are the same in size and capacity,
The difference lies in the method of treating the fine dust that accompanies the exhaust gas from the cyclone (2).

In the method of (1), as the operation continued, scale accumulated, the system clogged, the pressure loss increased, the treated air volume decreased, and the capacity became almost 1/2 in 20 days. Had to be cleaned.

In this method, the fine dust collected by the electrostatic precipitator is returned to the granule molding device as it is, so the particle size of the granules circulating in the device becomes finer, and the particle size of the granules supplied to the kiln also increases accordingly. It became fine and caused dusting, so I had to reduce the wind speed in the kiln and deal with it. (A decrease in the air flow results in a decrease in the amount of fuel used and a decrease in the amount of processing.) On the other hand, the method of the present invention does not cause the problem of blockage or dusting even during long-term operation and has a high efficiency. Was possible.

The results are shown in Table 1.

The weight and the average particle size (d ₅₀ ) of particles in step A (exit after heat treatment of the rotary kiln) and step B (exit the screw conveyor, are the water ejector inlet, and are the rotary valve inlet) are shown.

Comparing the methods, in the method, the solid B amount clogs the wet scrubber and the air amount of the entire treatment system is reduced, so that the treatment amount is reduced by 17% (20-day average).

Also, comparing the methods with each other, the method shows that the dust collected in the electrostatic precipitator returned to the granule molding device is fine (5 μm).
m; cf 63 μm), so the granules fed to the rotary kiln will eventually become finer (0.4 cm; cf 0.8 c
m), the air flow rate had to be reduced to prevent dusting in the rotary kiln, and the throughput of the entire system decreased.
(47% reduction) [Effect of the invention] As is clear from the examples, according to the method of the present invention, the amount of treatment is large and stable operation is possible. It does not change much, dusting can be prevented, the circulation amount of fine dust is small, and the load on the granule molding machine is also small. The particle size of the circulating fine dust is also large, and it is granulated up to 10 times the size of the method. Is advancing, and there is an effect such as the fact that this is reducing the circulation amount of fine dust.

As described above, the present invention is a method for removing carbon compounds in the circulating liquid of the buyer process, which improves the processing capacity of the present processing apparatus and enables stable operation for a long period of time.

[Brief description of drawings]

FIG. 1 is a flowchart showing one embodiment of the present invention. FIG. 2 is a conceptual diagram of the flow used in the example. 1,2: Cyclone 3: Electrostatic precipitator 3 ': Water ejector 4: Granule forming machine 5,6: Fan 7: Rotary kiln 8: Screw conveyor 8': Rotary valve

Claims (5)

[Claims] 1. A liquid to be treated is a circulating liquid of a buyer process in an alumina manufacturing process by the Bayer method, and a molar ratio of sodium and aluminum in the liquid to be treated or its concentrate is A.
₂ O ₃ / Na ₂ O is adjusted to a range of 1 to 5, the liquid to be treated and / or its concentrate is granulated, the granules are subjected to air classification, and large granules are heat treated at 500 to 1350 ° C. The remaining granules are returned to the granulation process and granulated again, while the gas after air classification is treated with a dry dust collector, and the collected fine dust is tumbled and compressed, and this fine dust is used in this process. A method for removing carbon compounds in a circulating liquid of a buyer process, characterized in that 2. When recovering uncollected fine dust in the classification step, it is recovered by an electric dust collector, and the recovered fine dust is rolled and compressed by a screw conveyor and returned to the main step. A method for removing a carbon compound in a circulating liquid of a buyer process according to claim 1. 3. Removal of carbon compounds in the circulating liquid of the buyer process according to claim 1, wherein the granules taken out from the granule molding equipment are heated by using a rotary kiln exhaust gas and used as a gas for air classification. Law. 4. A method for removing carbon compounds in a circulating liquid of a buyer process according to claim 1, which comprises using two cyclones for air classification and dry classification and dust collection. 5. A part of the granules taken out from the dry classification cyclone is sent to a rotary kiln, and the remaining granules and the granules collected by the dust collecting cyclone are circulated to a granulation molding facility. The method of claim 4 comprising: