WO2007009379A1 - A process for reducing the content of carbonates in the flow path of alumina production - Google Patents

Abstract

A process for reducing the content of carbonates in the flow path of alumina production, which comprises substituting the shaft kiln for rotary kiln for lime calcining in the production of aluminum oxide, wherein the content of carbon dioxide in the lime is 2~3%. Instead of the current sintering method for Bayer process alkali supplementing procedure, it employs liquid caustic alkali containing NaOH ≥ 42% to supplement alkali into the Bayer process system. The process can reduce the content of carbonate in the process, which would eliminate the negative affect of carbonate and improve the efficiency of heat exchanger.

Description

 Method for reducing carbonate in a process during alumina production Technical Field:

 The invention relates to an alumina production process, which is used for a lime burning process in an alumina plant raw material workshop and an alkali replenishing process in an evaporation plant.

Background technique:

 In the Bayer process alumina production process, the sodium carbonate in the process is continuously accumulated. After each Bayer cycle process, the sodium carbonate in the process accumulates a part of the increase, and the increase of the sodium carbonate content has many adverse effects on the production. Therefore, there are salt causticizing processes in production. The function of the salt causticization process is to precipitate sodium carbonate in the evaporation mother liquor from the solution, and convert the sodium carbonate into caustic soda by lime causticization, and recycle the causticized solution back to the process.

 The sodium carbonate in the process is mainly brought in by the following four routes:

(1) Bringing co ₂ into lime: Producing alumina with diaspore, 7-10% lime is added in the raw material grinding process, since lime usually contains 3 ~ 4% C0 ₂ , C0 ₂ Lime enters the process.

(2) Bauxite is brought into C0 ₂ : Bauxite usually contains a small amount of dolomite and other minerals containing co ₂ , and the ore is added to the raw mill to grind, and the co ₂ in the ore enters the process.

(3) Bringing C0 ₂ into the alkali supplement: In the production and filtration of alumina, it is necessary to continuously replenish the alkali in the process, whether it is liquid caustic or supplemental industrial alkali powder, or the sintering method to the Bayer process, the alkalinity must be co ₂ into the process, the liquid base containing less co _2, so that the process of co ₂ into less, and industrial base powder mainly to Na ₂ C0 _3, the C0 ₂ into the large, sintering solution The high carbonate content, this method of alkali addition will also bring a lot of carbonate into the process.

(4) The air contains C0 ₂ , and the C0 ₂ is in contact with the sodium aluminate solution for a long time, and a small portion of the NaOH in the sodium aluminate solution is converted into Na ₂ C0 ₃ , so that the C0 ₂ in the air enters the process.

Due to the above four aspects of co ₂ into the process, these co ₂ into the process, are in the form of sodium carbonate in the process of accumulation, resulting in high flow of sodium carbonate, adversely affect the production of alumina.

Processively, the conversion of caustic soda to Na ₂ C0 ₃ is detrimental to alumina production. Sodium carbonate is an ineffective base that circulates in the process and adversely affects alumina production, such as causing sodium aluminate solution. Increased viscosity, unfavorable for the production of sand-like alumina products, precipitation during the dissolution and evaporation process to form sodium carbonate crucibles, blockage of pipes, valves, adhesion to heat transfer The surface of the device reduces its heat transfer coefficient, affecting the normal operation of the heat exchanger.

Through the material balance calculation and analysis, the co _{2 in the} process is mainly brought into the production process through four ways, in which the co ₂ in the bauxite is determined by the characteristics of the ore itself, and the co ₂ in the air is inevitable, in the current technical conditions. Next, these two factors are unchangeable. In fact, these two factors is not much flow into C0 ₂ is usually only about ^30% and _<02 a greater amount of additional factors into ^two processes, i.e., a base of lime and complement factors, usually It accounts for about 70%, and these two factors can be improved.

By analyzing the co ₂ feed process, the present invention proposes a new process to reduce the concentration of carbonate in the solution.

Summary of the invention:

 The object of the present invention is to: use a new process to reduce the carbonate content of the process and eliminate the adverse effects of carbonate on the process.

The invention is realized by: converting the lime in the alumina furnace to the lime in the rotary kiln, and the C0 ₂ content in the lime is 2 to 3%; canceling the existing sintering method to the Bayer process , using NaOH > 42% liquid caustic to the base method of the Bayer process.

In the past alumina plants, all of them were fired in a shaft furnace, and the decomposition rate of the limestone in the shaft furnace was low, and the obtained lime had a high content of <:0 ₂ and was inferior in activity. The rotary kiln is used instead of the shaft furnace to burn lime. The decomposition rate of limestone can be increased from 88 to 91% to 96 to 98%, and the content of C0 _{2 in} lime is from 4 to 6 °/. Reduced to 2 ~ 3%, the C0 ₂ brought into the process by lime is greatly reduced.

 At present, five of the six largest alumina plants in China use a combined process. According to the theory of joint method, the base of the Bayer process uses the seed system of the sintering system. Because of the high content of impurities such as sodium carbonate and sodium sulfate in the mother liquor of the sintering method, the alkali method will use a large amount of carbon. The acid salt is brought into the Bayer process. The invention converts it into a Bayer process system to supplement the liquid caustic soda by NaOH > 42%, and does not use the sintering method to separate the mother liquor to supplement the alkali, thereby eliminating the influence of the carbonate in the mother liquor of the sintering method on the Bayer system.

 The beneficial effects of the invention are as follows: the carbonate content in the process is reduced, the adverse effect of the carbonate on the process is eliminated, and the working efficiency of the heat exchanger is improved.

detailed description:

Embodiments of the present invention: The existing shaft furnace fired lime in the alumina plant is changed into rotary kiln burnt lime, the lime burned in the rotary kiln is good in quality, and the lime content in the co _{2 is} low; for the combined alumina plant, Bayer The system system uses alkali hydroxide with NaOH > 42%, and does not use the seed system of the sintering system to supplement the alkali. By adopting these two measures, the Co ₂ process brings in co ₂ Can be greatly reduced.

 The lime burning process of the alumina plant is currently disposed in the raw material workshop of the alumina plant. From the perspective of environmental protection, the lime burning process can be arranged in the alumina plant by using the rotary kiln to burn lime, or the lime can be fired. The process is arranged in a limestone mine, and the burnt lime is transported into the alumina plant by a car for use in an alumina plant.

Claims

Rights request 1. A method for reducing carbonate in a process in an alumina production process, characterized in that: the alumina plant is fired in a shaft furnace and the lime is burned in a rotary kiln, and the co ₂ content in the lime is 2 to 3 %; using NaOH > 42% liquid caustic to the base method of the Bayer process.