JP3440483B2 - Method for removing water from circulating alkali aluminate solution - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing excess water from a circulating alkaline aluminate solution in a buyer process, and more specifically, it is possible to prevent scaling in an evaporator even during long-term continuous operation. It relates to a method of removing excess water from a remarkably small amount of circulating alkaline aluminate solution.

[0002]

The most commonly used method for producing aluminum hydroxide from alumina-containing ores is the Bayer process. This method is an alumina-containing ore, for example bauxite is treated with an alkaline aqueous solution such as caustic soda or a mixed solution of caustic soda and sodium carbonate to form a slurry, and the alumina content in the bauxite is extracted as an alkali aluminate (extraction step), Thereafter, insoluble residues such as iron oxide, silicate, and titanium oxide are separated from the slurry (red mud separation step), and aluminum hydroxide is added as seeds to the clear alkaline aluminate solution after separating the insoluble residues. Usually, aluminum hydroxide is precipitated at a temperature of about 50 ° C to about 75 ° C, the precipitated aluminum hydroxide is separated from the alkaline aluminate solution (precipitation step), and a part of the separated precipitated aluminum hydroxide is circulated for use as seeds. Then, the remaining aluminum hydroxide is taken out as a product, while the separated aluminum aluminate solution (minute Liquid) is constituted of a step of recycling the intact, or the extraction of alumina from alumina-containing ore was concentrated.

In the buyer process, usually, when aluminum hydroxide is taken out as a product, the purity of the product is increased,
Aluminum hydroxide is washed with warm water for the purpose of recovering the attached decomposition liquid. Further, the insoluble residue of the alumina-containing ore is also washed with warm water to collect the alkaline aluminate solution adhering to the insoluble residue before discharging it out of the system. These wash waters are allowed to flow into the process (in the circulating alkaline aluminate solution) for the purpose of recovering the alkali. In addition, water adhering to bauxite, water for washing plant equipment, water accompanied by a supplemental alkali source, condensed water generated by blowing live steam for the purpose of heating in the extraction process, etc. flow into the process.

On the other hand, water is discharged from the alumina manufacturing process by adhering to the product aluminum hydroxide and bauxite dissolution residue, but the amount of water entering the alumina manufacturing process is large and the circulating alkaline aluminate solution It is necessary to evaporate surplus water to keep the alkali concentration constant.

As one of the evaporation operations, in the step of extracting alumina from bauxite, the slurry after pressurizing and heating is also used for the purpose of heat recovery by flushing. It is unbalanced, requires additional evaporation, and has traditionally used multiple-effect vacuum evaporators and multi-stage flash processes with large evaporation ratios (eg, "aluminum industry").
Kitagawa Jiro, Seibundo Shinkosha, page 82 6. See the section on adjusting the concentration of the solution, and the Chemical Engineer June 1970 page 156- (THE CHMICAL ENGINEE
R, JUNE, 1970) ALUMINA PRODUCTION; PRINCIPLES AND PRAC
TICE, especially on page 162 Water Cycle).

As is well known, in the multi-stage flash method, the evaporation amount is obtained by the following equation. (Multi-stage flash method) Evaporation amount = circulating liquid amount × (t ₁ −t ₂ ) / enthalpy (in the formula, t ₁ ; circulating liquid temperature entering the first-stage evaporator, t ₂ ;
The final stage shows the circulating fluid temperature exiting from evaporator) Therefore, in order to increase the evaporation amount is not necessary to increase the (t ₁ -t _2), vapor in the evaporator of the last stage has a low potential cooling Must be cooled with water. Industrially, from the economical point of view, it is common to use seawater or the like as the cooling water, and in a normal buyer process, for example, a circulating alkali aluminate solution at about 70 ° C. is used at about 120 ° C.
A method of heating to about 140 ° C, passing through 3 to 7 stages of evaporators, and then flashing to about 50 ° C in the last stage of evaporators is adopted, and the same operation is performed even when using a multi-effect vacuum evaporator. It is a condition.

[0007] However, although such a method uses less steam as compared with the single can evaporation and is superior from the economical point of view, it relates to the inner wall of the first or second high temperature side evaporator and the related method. Since the scale mainly composed of sodium aluminosilicate adheres to the heat exchanger and the heat transfer coefficient is remarkably reduced, the equipment must be stopped every few months and the removal operation by acid cleaning etc. must be performed. It has a drawback. In addition, in the evaporator on the low temperature side, there is a drawback that scale deposition due to sodium phosphate occurs, which causes a problem of blocking the outlet pipe leading to the evaporator and the heat exchanger. In addition, the steam absorbed by the cooling water in the final-stage evaporator is discharged as hot waste water to the outside of the system, resulting in energy loss.

[0008]

In view of such circumstances, in view of the above circumstances,
The present inventors remove excess water from a circulating alkaline aluminate solution that has significantly less scale adhesion due to sodium aluminosilicate or sodium phosphate, is capable of continuous operation for a long time, and is also economical As a result of diligent studies to find out a method, the inventors have found that all the problems can be solved when a specific evaporation method is used under specific conditions, and have completed the method of the present invention.

[0009]

[Means for Solving the Problems] That is, according to the method of the present invention, an alumina-containing ore and an alkaline solution are mixed to form a slurry, and alumina is extracted from the ore as an alkaline aluminate solution, and the insoluble residue is extracted from the slurry. After separation and removal, aluminum hydroxide is precipitated from an alkaline aluminate solution, filtered, and the remaining solution is circulated and used as an alkaline solution for extracting alumina from an alumina-containing ore, a so-called Bayer method for producing aluminum hydroxide. In order to remove excess water accumulated in the circulating alkaline aluminate solution, a vapor compression evaporation method is used to remove water from the circulating alkaline aluminate solution.

The method of the present invention will be described in more detail below.
The vapor compression evaporation method applied to the method of the present invention may be any known one, for example, Handbook of Chemical Engineering (Chemical Engineering Handbook, published by Maruzen Co., Ltd., 3rd edition), p. Section and Chemical Engineering Theory (Hario Hikita, Asakura Shoten, 13th edition, 1979) Chapter 4 Evaporation, page 109-
P. 110, "4.4.4 Vapor compression evaporation method",
It is usually composed of an evaporator, a vapor compressor and a heat exchange part, which compresses the vapor generated from the evaporator to raise its temperature,
This is configured to be sent to the heating chamber of the evaporator and used as a heat source for heating to take out condensed water. The evaporator and the vapor compressor may be single, or plural may be used.
Further, normal pressure evaporation or reduced pressure evaporation may be adopted.

In the present invention, the temperature of the circulating alkali aluminate solution in the evaporator of the vapor compression evaporation method is about 60 ° C. to the boiling point temperature under normal pressure, preferably about 70 ° C. to about 100 ° C. When the temperature of the solution in the evaporator exceeds the boiling point temperature under atmospheric pressure (about 106 ° C.), the scale mainly composed of sodium aluminosilicate adheres frequently. In particular, the precipitation rate of sodium aluminosilicate in a solution is highly temperature-dependent, and the higher the solution temperature, the faster the precipitation exponentially. On the other hand, at lower temperatures, scale is formed due to sodium phosphate. The scale derived from sodium phosphate has a dissolution characteristic as shown in FIG. 1, and the higher the alkali concentration (Na ₂ O) and the lower the solution temperature, the smaller the solubility. Therefore, for example, the alkali concentration in the circulating alkali aluminate solution is 130 N by the concentration operation.
a ₂ Og / l to 160 Na ₂ Og / l, when using solution temperatures above about 60 ° C. in the evaporator and heat exchanger, the scale of sodium phosphate caused The problem can be virtually eliminated.

The vapor compression evaporator can be installed at a place where a multi-effect vacuum evaporator or an evaporator using a multi-stage flash method has been installed in the past. Usually, the decomposition liquid separated from the aluminum hydroxide precipitation step is used. It may be installed in a process until it is reused for melting the alumina-containing ore.

When the method of the present invention is adopted, it is also possible to recover and remove sodium fluoride from the circulating alkaline aluminate solution, if necessary. It is conceivable that the accumulation of fluoride in the solution may be mixed with the aluminum hydroxide that precipitates, causing variations in the desired alumina quality during the firing process. For example, it is known that fluoride is used as a mineralizer to adjust the shape of crystal grains in the firing of alumina, but unadjustable fluoride is mixed in the raw material aluminum hydroxide with a change in amount. In that case, it becomes difficult to adjust the crystal grain shape during firing. It is also considered that these fluorides corrode the materials for the firing equipment such as bricks and steel materials.

When the method of the present invention is adopted and the evaporation operation is carried out within the boiling point temperature range of about 60 ° C. to atmospheric pressure, the inner wall of the evaporator (the heat exchange related to it) is concentrated under the concentrating condition of a specific alkali concentration. The scale caused by sodium fluoride can be generated and adhered to (including the wall surface of the vessel). Therefore, if it is intended to remove the fluoride from the inside of the buyer process, the device structure may be such that the scale is attached to the inner wall of the evaporator to a certain amount and then washed out with water. Also, even under concentrated conditions where scales due to sodium fluoride are generated on the inner wall of the evaporator, sodium fluoride is water-soluble, so if the scale-adhered surface such as the inner wall of the evaporator is regularly washed with water. In normal operation, continuous operation is possible without causing a decrease in heat transfer coefficient due to scale adhesion.

The vapor compression evaporation method applied to the method of the present invention has a higher equipment cost than the multi-effect vacuum evaporator having the same evaporation capacity and the multi-stage flash method, but the energy consumption rate per evaporation amount is remarkably low, so that the production cost of alumina is low. Is superior to the conventional method.

[0016]

According to the method of the present invention described in detail above, the multi-effect vacuum evaporator and the multi-stage flash method, which have been conventionally used for removing surplus water from a circulating alkaline aluminate solution, are replaced by vapor compression evaporation. By adopting the method and performing the operation by the evaporation method at a specific temperature, the adhesion of scale due to sodium aluminosilicate or sodium phosphate to the inner wall of the evaporator is extremely small, and continuous operation for a long time is possible. In addition, it is possible to remove sodium fluoride from the circulating alkaline aluminate solution if necessary, and in addition to being economical, a method for removing excess water from the circulating alkaline aluminate solution has been found. The industrial value is enormous.

[0017]

EXAMPLES The method of the present invention will be described in more detail with reference to the following examples, which should not be construed as limiting the present invention.

Example 1 A vapor compression type evaporation can (total heat transfer area 1900 m ² ) consisting of one can and divided into four chambers had a Na ₂ O concentration of 130 g /
l, circulating sodium aluminate solution 125 at a temperature of 80 ° C
m ³ / Hr was continuously supplied, and 21 m ³ / Hr of water was evaporated under the condition that the liquid temperature in the evaporator was 80 ° C. (the solution temperature after the evaporation treatment was 80 ° C., the Na ₂ O concentration was 156 g / l). During operation For the purpose of preventing a trace amount of NaF from adhering to the heat transfer surface of the heat exchanger in the evaporator, continuous washing with hot water is carried out continuously for 1 hour and 1 room / 4 days, and operation is continued for 1 year and 3 months. Although it continued, no trouble due to scale adhesion and no decrease in thermal efficiency were observed during this period.

Example 2 Except that the heat exchanger in the evaporator is not regularly washed with hot water,
Using the same evaporator and circulating sodium aluminate solution as in Example 1, liquid was continuously supplied at 70 m ³ / Hr, and 21 m ³ / Hr of water was evaporated under the condition of the liquid temperature in the evaporator of 80 ° C. ( The solution temperature after the evaporation treatment was 80 ° C., the Na ₂ O concentration was 170 g /
l). After continuously operating for 2 weeks under such conditions, the evaporator was dismantled and the heat transfer surface was about 1.5 mm NaF.
Scale was attached to the entire surface. Next, as a result of washing the heat transfer surface with warm water, about 7 tons of NaF could be easily removed.

Comparative Example 1 Na ₂ O concentration of 130 g / l was added to a 5-stage flash type evaporator.
Circulating sodium aluminate solution at a temperature of 80 ° C 125m ³ /
By continuously supplying Hr and operating the inlet temperature of the first-stage flasher at 128 ° C and the outlet temperature of the fifth-stage flasher at 50 ° C, 21 m ³ / Hr of water was evaporated (after evaporation treatment). Solution temperature is 50 ° C., Na ₂ O concentration is 156 g / l).
When operated continuously for 3 months under such conditions, descaling scale is generated on the surface of the heat exchanger that heats the solution in the first-stage flasher to 128 ° C, and the heat transfer coefficient is 2000 Kcal / m. ° C. Hr to 1000 Kcal / m.
Since the temperature decreased to ℃ .Hr and the prescribed amount of evaporation could not be maintained, the reactor was stopped and the scale was removed by acid washing. In addition, the outlet pipe from the fifth flasher was clogged with a pipe diameter for scale production of a sodium phosphate system, so it was washed and removed with hot water every month.

[Brief description of drawings]

FIG. 1 is a diagram showing the solubility of sodium phosphate in a circulating alkaline aluminate solution.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-61-251513 (JP, A) JP-A-54-32171 (JP, A) JP-A-62-279801 (JP, A) JP-A-1- 27601 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) C01F ^7/ 00-7/76 B01D 3/00-3/42

Claims (4)

(57) [Claims] 1. An alumina-containing ore and an alkaline solution are mixed to form a slurry, and alumina is extracted from the ore as an alkaline aluminate solution, and the insoluble residue is separated and removed from the slurry, and then water is removed from the alkaline aluminate solution. precipitated aluminum oxide, allowed filtered at residual solution to the preparation of water aluminum oxide you recycled as an alkaline solution for alumina extraction from alumina-containing ore is accumulated in the circulating alkali aluminate solution in A method for removing water from a circulating alkaline aluminate solution, characterized by using a vapor compression evaporation method for removing excess water. 2. The circulating alkaline aluminate solution according to claim 1, wherein the temperature of the circulating alkaline aluminate solution in the evaporator in the vapor compression evaporation method is 60 ° C. to the boiling point temperature under normal pressure. Water removal method. 3. Mixing ore containing alumina and alkaline solution
Aluminic acid from the ore
Extract as an alkaline solution and remove the insoluble residue in the slurry.
After separation and removal, hydroxylation from alkaline aluminate solution
Precipitate and filter the aluminum, and then wash the remaining solution with aluminum.
Alkaline Solution for Extracting Alumina from Na-Containing Ore
It is a manufacturing method of aluminum hydroxide that is recycled after being used.
The excess accumulated in the circulating alkaline aluminate solution.
Characterized by using vapor compression evaporation method for water removal
A method for producing aluminum hydroxide. 4. Circulation in the evaporator for vapor compression evaporation
The temperature of the alkali luminate solution is 60 ° C to the boiling point temperature under normal pressure.
The manufacturing method according to claim 3, wherein the manufacturing method is a degree.