KR800000592B1 - Process for recovering caustic alkali from spent alkali liquer - Google Patents

Abstract

In a process for recovering caustic alkali from spent alkali liquor wherein the spent alkali liquor is roasted with iron oxide to obtain a roasted product which contains alkali ferrate, followed by hydrolysis of the roasted prooduct, the improvement comprising: reacting the roasted product with water at a pemperature not lower than 110≰C, and under sufficient pressure to maintain the reaction system in liquid phase; and separating the resultant aqueous caustic alkali soln.

Description

How to recover caustic alkali from alkaline waste liquid

1 is a graph showing the relationship between the concentration of sodium hydroxide in an aqueous solution of hydrolyzed alkali ferric acid salt at 100 ° C. or lower, yield of hydrolysis or decomposition, and dissolved iron content at normal pressure.

Figure 2 is a graph showing the results of hydrolysis at 110 ℃ or more under pressure.

The present invention relates to a method for recovering caustic alkali from alkaline waste liquor, and more particularly, to an improved method for recovering high purity and high concentration of caustic alkali from alkaline waste liquor by hydrolyzing a small product of iron waste and iron oxide.

The term “alkali waste liquid” as used herein refers to waste liquids generated from various chemical processes in which pulp is manufactured using caustic alkali, and alkali-containing waste liquids discharged from standard processes are treated with alkalis. It means the waste liquid discharged from the process of treatment with alkali.

It is well known that in recovering caustic alkali from alkaline waste liquids released by various chemical industries, the alkaline waste liquid is burned with iron oxide and then the product is hydrolyzed below 100 ° C and under normal pressure.

For example, Japanese Patent Application No. 49-21631 uses caustic alkali to burn and melt an alkaline waste liquid discharged from the manufacture or bleaching process of fibrous material with iron oxide at a temperature of 700 to 900 ° C and melt the resulting melt in water. Processes are added dropwise and dissolved at 50 to 100 ° C. to obtain an aqueous solution of sodium hydroxide.

In Japanese Patent Application No. 51-154920, waste organic acid materials such as phenol and carboxylic acid produced in various chemical processes are reacted with caustic alkali solution, neutralized, extracted and adsorbed to remove organic alkali salt solution, and oxidizing atmosphere of 800 ℃ or higher. The process of burning an organic alkaline salt solution together with ferric oxide to polish the ash, leaching caustic alkali with water, and separating it from the precipitation consisting mainly of ferric oxide is shown.

The caustic alkali recovery process described in the above two Japanese patent applications is based on the following reaction.

M ₂ CO ₃ + Fe ₂ O ₃ = M ₂ Fe ₂ O ₄ + CO ₂ . … … … (One)

M ₂ O + Fe ₂ O ₃ = M ₂ Fe ₂ O ₄ . … … … (2)

MeFe ₂ O ₄ + H ₂ O = 2MOH + Fe ₂ O ₃ . … … … (3)

Where M is an alkali metal.

Upon combustion, the alkaline waste liquid having the above-mentioned organic components is converted into alkali carbonates and / or alkali oxides and simultaneously undergoes reactions of (1) and (2) to produce alkali ferrates. According to reaction (3), alkali ferric acid salt is hydrolyzed at 100 degrees C or less to obtain caustic alkali and ferric oxide.

However, the rate of hydrolysis in the above mentioned process is unexpectedly particularly slow and the alkali concentration produced is relatively high. When the alkali concentration is 10% or more, the yield of hydrolysis at 100 ° C or lower is at most 60 to 70%.

The loss of the non-recovery content of caustic alkali obtained in the conventional hydrolysis state in the known process can be prevented by recycling the alkali ferrate residue which has not undergone hydrolysis. However, recycling large amounts of ferric acid residues requires extensive equipment and is unreasonable in terms of energy economy. Therefore, the rate of hydrolysis should be as fast as possible.

Usually, the yield of hydrolysis is further lowered when the recovery solution has a drawback containing relatively high concentration of molten iron when the aqueous solution of a high concentration of caustic alkali of 15% or more is a substantial value. The presence of large amounts of molten iron in the dilute alkaline aqueous solution limits the use of the solution or causes unnecessary reactions or contamination during use of the recovered alkaline solution. Therefore, it is advantageous to minimize the iron content in the dilute alkaline aqueous solution. Curves a and b in FIG. 1 are mixtures of iron oxides and ferric acid salts obtained by burning a mixture containing iron oxide and sodium carbonate having a molar ratio of 1.3: 1 (adding sodium carbonate and the same amount of starch as organic matter) at 950 ° C. for 1 hour. The phases respectively show the relationship between the concentration of sodium hydroxide in aqueous solution, the rate of hydrolysis and the molten iron concentration in the aqueous solution during the conventional process of hydrolysis for 3 hours at 65 ° C. and 0.5 hours at 98 ° C., respectively. The drawbacks of conventional processes are evident in this graph.

FIG. 2 is a graph similar to that of FIG. 1 but shows the results of hydrolysis of a mixture of iron oxide and sodium ferrate under the conditions according to the invention. In particular, curves c to g are the results of hydrolysis at 110 ° C. and 150 ° C. for 0.5 hours, 150 ° C. for 5 minutes, and 180 ° C. for 2 minutes. Curve b at 98 ° C./0.5 hour in FIG. 1 is shown by the dotted line for comparison. It can be seen in the graph of Table 2 that unexpected results are obtained by reactions performed at 110 ° C. or higher. This effect is more evident when using sodium hydroxide concentrations of 15% by weight or more.

The superiority and inherent advantages of the process of the invention in which the hydrolysis is carried out in the liquid phase at 110 ° C. or higher, under pressurized conditions as compared to conventional processes, become more apparent with the following description of FIGS. 1 and 2.

In general, the high concentration of caustic alkali at the end of the hydrolysis reaction of alkali ferrates under a given temperature condition means a low conversion of hydrolysis as shown in FIG. 1 and FIG. In contrast to conventional processes (Figure 1) where the hydrolytic conversion does not exceed 60 to 70% by performing below 100 ° C, the method of the present invention produces a high concentration of caustic alkali solution in which at least 80% or more is converted ( 2). Although not shown in FIG. 2, when performed at 130 ° C. or higher, it converts to a high rate of 90% or higher. As can be clearly seen in FIG. 2, the conversion is increased in proportion to the temperature and the hydrolysis is performed almost completely at 180 ° C. even when a high concentration of caustic alkali solution is produced. It can be seen from the curves e and f, which represent the conversions carried out by hydrolysis at 150 ° C. for 30 and 5 minutes respectively, that the rate of hydrolysis under conditions according to the invention is extremely high.

Regarding the concentration of molten iron, the concentration increases with temperature at hydrolysis below 100 ° C. (FIG. 1). In particular, the concentration of molten iron rapidly increases to about 15% by weight of caustic alkali concentration in the final product, as can be seen in FIG. In contrast, when the hydrolysis reaction is carried out under temperature conditions of 110 ° C. or higher according to the present invention, the concentration of molten iron in the final aqueous solution tends to be inversely proportional to the temperature as shown in FIG.

Increasing the reaction temperature according to the method of the present invention can suppress the concentration of molten iron to 50 mg / L or less, and can also be suppressed to about 10 mg / l.

In addition, the increase in the caustic alkali concentration in the recovery solution only performs a small increase in the molten iron compared with the case shown in FIG.

As can be seen from the above it is important to perform the hydrolysis at 110 ℃ or more, preferably 130 to 180 ℃ or more. The upper limit of the theoretically feasible reaction is the critical point at which the solution can remain liquid and rise above the critical temperature of water due to the presence of caustic alkali. However, it is practical to have a temperature upper limit of 300 ° C. or less, preferably 250 ° C., in view of pressure, corrosion, material properties, and performance cost.

The fermentation time of the hydrolysis reaction depends on the desired caustic alkali concentration and its yield as well as the reaction time and the type of reactor used. In general, coronary reactors require less time than autoclave to achieve the same results. In order to substantially terminate the reaction, a high time of several seconds to several tens of minutes is sufficient at a high temperature, but when the treatment is performed for a longer time, the color of the reaction liquid is much better. In particular, the reaction liquid which has been treated for a short time has a light yellowish green color, but when treated at a high temperature for a long time, a reaction liquid which is light yellow or almost colorless is produced. The fermentation time should be determined by the purpose for which the reaction liquid is used or other conditions.

Pressure is applied during the hydrolysis reaction according to the invention to keep the reaction system in the liquid phase. In order to hydrolyze the alkali ferrate to obtain an aqueous solution of caustic alkali, an appropriate amount of water is added in advance to the desired concentration in consideration of the amount of the alkali ferrate to be hydrolyzed. At temperatures above 110 ° C, water is lost due to a significant amount of evaporation and it is difficult to maintain the liquid phase of the reaction system, and thus pressurization is necessary. The pressure to be applied varies depending on the final concentration of the reaction temperature caustic alkaline solution or other conditions. The pressure required to maintain the above-mentioned state when raising the temperature from 110 ° C to 300 ° C varies between 110 and about 100 atm. Normally it is appropriate to maintain the liquid phase at 2 to 8 atm at a temperature of 130 to 180 ° C. using 50 atm or lower pressure. Pressurization is carried out using the vapor pressure of the reaction system or by inert gas such as nitrogen gas or air from which carbon dioxide is removed.

The alkali in the waste liquid treated by the process of this invention contains alkali metal salts, such as sodium, lithium, and potassium. When the waste liquid is mixed with a powder of iron oxide (Fe ₂ O ₃ ) and heated at 800 ° C. or higher, the alkali in the waste liquid is converted into alkali carbonate and / or alkali oxide and alkali ferrate. It is practical to use an excess of iron oxide, usually to reduce the amount of alkali carbonate that remains unreacted with iron oxide in the manufacturing process and to increase the reaction rate. It is desirable to sufficiently powder the mixture of alkali ferrate and excess iron oxide prior to hydrolysis to improve contact with water and to increase the reaction rate. However, even without powdering, iron oxide is gradually released from the surface in order to proceed with the hydrolysis reaction for a long time.

The reactor used in the process of the present invention is a continuous autocrab or batch that heats or cools or stirs the material. In addition to the tubular pressure resistant reactor, an oil phase type, a fluidized bed type, a multistage type, or a type in which the holding time is ensured by the recycling of the reaction is used. The raw materials are placed in a reactor together with water to carry out a hydrolysis reaction under predetermined conditions. The reaction is, of course, carried out in an atmosphere free of carbon dioxide. The hydrolysis product consists of a mixture of fine particles of iron oxide and an aqueous solution of caustic alkali in which each substance can be separated using a precipitation vessel, filter, decanter or other suitable separation means.

It can be seen from the above that caustic alkali according to the method of the present invention can be easily obtained from alkali ferrates at high concentrations that can meet practical requirements, while reducing the content of molten iron and other impurities. The process of the invention is particularly advantageous when recycling caustic alkali to recover from alkali containing organic waste material.

The present invention is also applicable to the hydrolysis of the molten product produced by combustion of a mixture of sodium carbonate and iron oxide powder in order to convert sodium carbonate to sodium hydroxide.

Example 1

A mixture of Fe ₂ O ₃ , Na ₂ CO ₃ and starch in a weight ratio of 1.96: 1: 1 (molar ratio of Fe ₂ O ₃ and Na ₂ CO ₃ = 1.3) was placed in an alumina crucible as an alkaline waste solution at 950 ° C. in the presence of air. It is calcined in an electric furnace for 1 hour to obtain the calcined product. The product is cooled under atmospheric pressure to remove carbon dioxide and powdered to a size of 100 mesh or less under inert gas pressure. The powder is used as a sodium iron sample.

7 to 8 g of ferric acid treatment and 6 ml of water are placed in a Nickel wire autoclave with an internal capacity of 15 ml and purged with nitrogen gas and pressurized at 10 atm to prevent loss of water under the reaction conditions. The reaction is carried out at 150 ° C. with stirring of the autoclave for a predetermined time. After cooling, the contents are removed from the autoclave and filtered. The filtrate is analyzed to obtain the following results.

Comparative Example

The same sample of 8 and 6 ml of water as used in Example 1 were placed in an autoclave of the same type as in Example 1. After replacing the air in the autoclave with nitrogen gas, the reaction proceeds at 98 ° C. for 30 minutes.

The test results are shown below.

Example 2

A mixture of cyclohexanone and cyclohexanol, obtained by oxidizing cyclohexane with air using an oxidizer, was extracted with 23% aqueous sodium hydroxide solution and extracted with sodium salt of organic acid having the following analysis (alkali waste solution). Get

Analytical value of extract C: 18.5% (weight)

H: 2.9%

O: 17.1%

Na: 9.3%

H ₂ O: 52.2%

2.44 parts of the extract is mixed with 1 part of iron oxide (molar ratio of Fe ₂ O ₃ and Na ₂ O = 1.27), shaken well and evaporated to dryness. The product is placed in a kiln-type rotary furnace and burned at about 1,000 ° C. while burning auxiliary fuel in an amount of 1.4 parts per 100 parts of extract. The run time is about 30 minutes. The black off-product is wet polished under fire, followed by the addition of 99 parts of water per 100 parts of product to obtain a slurry. The slurry is hydrolyzed in a tubular reactor at 150 ° C. for 5 minutes.

The aqueous solution obtained by separating the iron oxide by filtration had a yield of hydrolysis of 99% and showed the following analysis.

Analysis

NaOH 23.0%

Na ₂ CO ₃ 1.6%

Molten Iron 19mgFe / ℓ

Claims (1)

Under the conditions of the temperature of 110 ° C. or higher and pressurized conditions under which the reaction system can be maintained in the liquid phase, an alkaline waste liquid is combusted together with iron oxide, characterized by separating the caustic alkaline waste liquid generated after reacting the small product with water to produce alkali ferric acid salt. A method for recovering caustic alkali from an alkaline waste liquid by obtaining a bovine product containing and then hydrolyzing the bovine product.

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