Cause analysis and improvement measures of high sodium carbonate content in caustic soda

Chlor-Alkali Plant A is a salt chemical enterprise mainly engaged in the production of basic chemical raw materials such as chlor alkali and downstream chlorine products. Adopting the bipolar ion membrane process for alkali production, there are currently 7 electrolytic cells with a caustic soda production capacity of 170000 t/a.Recently, some customers have reported that the content of sodium carbonate in high-purity sodium hydroxide is too high. In response to this issue, the company has conducted in-depth analysis, identified the reasons from analysis, process, storage, transportation and other aspects, proposed targeted measures, and made improvements.

1. Current Status of Quality Analysis in the Caustic Soda Industry

After the problem of high sodium carbonate content in caustic soda emerged, through communication with other companies in the industry, most customers signed contracts with the company in the form of solid alkali. The existing analysis method only analyzes the total alkali content in the caustic soda solution and does not deduct the content of sodium carbonate in the caustic soda solution. Therefore, the analysis result is the total alkali content in the caustic soda solution.

The analysis of NaOH content in existing caustic soda adopts the neutralization reaction between sodium hydroxide and hydrochloric acid, with phenolphthalein as an indicator to indicate the endpoint. The reaction equation is NaOH+HCl→NaCl+H₂O.

2 Quality standard for high-purity sodium hydroxide

The quality standard for high-purity sodium hydroxide refers to the specified indicators for liquid caustic soda in the China national standard GB/T11199-2006. According to the standard, the highest content of sodium  carbonate in high-purity sodium hydroxide of first-class quality cannot exceed 0.06%.

3 Determination of Alkali Content in Caustic Soda

In China standard GB/T4348.1-2013 《Determination of Sodium Hydroxide and Sodium Carbonate Content in Industrial Sodium Hydroxide》, the method for determining the content of sodium hydroxide and sodium carbonate in industrial sodium hydroxide is specified. The content of sodium hydroxide in the sample solution is analyzed first, and then the total alkali content of sodium hydroxide and sodium carbonate in the sample solution is measured. The difference between the two is used to obtain the content of sodium carbonate in the sample solution.

3.1Principle of determination of sodium hydroxide content

Add barium chloride to the sample solution, convert sodium carbonate into barium carbonate precipitate, and then use phenolphthalein as an indicator to titrate with hydrochloric acid standard solution to the designated point. The reaction is as follows.

Na₂CO₃+BaCL₂→BaCO₃↓+2NaCL

NaOH+HCL→NaCL+H₂O

3.2Principle of determination of sodium carbonate content

The sample solution is indicated by a mixture of bromocresol green—methyl red, and titrated with hydrochloric acid standard solution to the endpoint. The total amount of sodium hydroxide and sodium carbonate is measured, and then the sodium hydroxide content is subtracted to obtain the sodium carbonate content.

4 Common solubility of sodium hydroxide and sodium carbonate

After checking the 《Inorganic Volume of the Chemical and Chemical Property Data Manual》, the common solubility of sodium hydroxide and sodium carbonate is shown in Table 1.

According to the data in Table 1, when the mass fraction of NaOH in the solution is constant, the Na₂CO₃ content in the solution increases with increasing temperature. Therefore, it is necessary to maintain the temperature of the NaOH solution stable at room temperature.

5 Analysis of the reasons for the high content of sodium carbonate in caustic soda

Aimed at the issue of high sodium carbonate content, based on the production process of caustic soda, the root cause of the problem is identified from the aspects of production, equipment, storage, transportation, and analysis.

5.1 Production process of caustic soda

Chlor-Alkali Plant A adopts the full brine alkali production technology for the production of primary brine. The saturated brine from the salt mine is refined by adding sodium hydroxide and sodium carbonate through a pre-processing unit, and then refined by a tower reactor and a Kaimo filter to remove calcium ions, magnesium ions, natural organic matter, and water-insoluble impurities from the brine, producing qualified primary brine.

First brine will be refined twice through a chelating resin tower to remove calcium and magnesium ions, producing refined brine that meets the requirements of ion membrane electrolysis process. There are a total of 3 chelating resin towers, 2 of which are in series operation and 1 is regenerated for backup. Switch once every 24 hours.

The electrolysis system adopts zero pole distance bipolar ion membrane electrolysis technology. The refined brine is sent into the anode liquid inlet manifold of the electrolysis cell through the brine head tank. After mixing with hydrochloric acid, it enters the anode chamber of the electrolysis cell, producing chlorine gas and diluted brine. The diluted brine is dechlorinated and returned to the salt mine. The alkaline solution is sent to the alkaline inlet manifold of the electrolytic cell through the head alkali solution tank, and diluted with pure water to maintain the concentration of caustic soda in the cathode solution at around 30%. The diluted caustic soda is sent to the cathode inlet manifold of each electrolytic cell and enters the cathode chamber of the electrolytic cell, producing hydrogen gas and caustic soda. Part of the caustic soda is measured by a flow meter and sent out of the boundary area as a finished product, while the rest is returned to the head alkali tank after heat exchange in the catholyte heat exchanger.

5.2 Analysis of the reasons for the high content of sodium carbonate

Based on the electrolytic production process and the current storage status of finished alkali products, the preliminary analysis shows that the reasons for the high content of sodium carbonate in the alkali solution are as follows.

To improve current efficiency and reduce electricity consumption per ton of alkali, it is necessary to ensure that calcium ions in the brine are completely removed. There are two steps to remove calcium ions from brine. In the first brine production step, sodium carbonate needs to be added to the brine to remove the majority of calcium ions. In order to reduce the production load of the secondary brine resin tower device and effectively remove Ca²⁺, the amount of refined agent sodium carbonate added should be slightly more than the theoretical amount required for the reaction. In actual process control, the excess of sodium carbonate in brine is 0.25-0.40g/L.

In the secondary saltwater production process, the removal of calcium ions is mainly carried out through a chelating resin tower. After the chelating resin adsorbs calcium and magnesium ions, the saltwater enters the electrolytic cell for electrolysis. Excess sodium carbonate in a saltwater solution enters the electrolytic cell, where it reacts with hydrochloric acid at the anode. Part of the unreacted carbonate ions reverse osmosis through the ion membrane to the cathode, forming sodium carbonate in the catholyte.

The caustic soda in the finished product tank area and the caustic soda in the caustic soda tank car, due to contact with air, reacts with carbon dioxide in the air to produce sodium carbonate, which dissolves in the sodium hydroxide solution.

To verify the above judgment, samples were taken from the cathode outlet of the electrolytic cell and the finished alkali tank for analysis.

When sampling at the cathode outlet of the electrolytic cell, considering the different operating cycles of the ion membrane in each electrolytic cell, two representative electrolytic cells were selected for comparative analysis during the experiment. The G cell with the longest membrane operating time and the F cell with the shortest membrane operating time were respectively selected. The analysis results are shown in Table 2.

From the above data analysis, it can be seen that the sodium carbonate content in the outlet alkali solution of tank F is 1/3 of that of tank G, and the sodium carbonate content in the outlet alkali solution of a single tank meets the national standard requirements of the product. This indicates that although the ion membrane of the electrolytic cell is a cation selective membrane, as its operating cycle extends, the density of the ion membrane decreases, and the ability to prevent anion reverse osmosis also decreases accordingly. This causes sodium carbonate in the anode solution of the electrolytic cell to enter the cathode solution through the ion membrane, but it is not the main reason for the high content of sodium carbonate in the finished alkaline solution.

When sampling in the finished alkali tank area, due to the low inventory of finished alkali in the tank area, it is not loaded at night, and there is continuity in loading during the day. Therefore, the contact time between the inside of the alkali tank connected to the electrolytic outlet and the air will not exceed 12 hours. On September 13th, the alkaline solution from tank 7 was selected for analysis, and the analysis results showed that the sodium carbonate content was 0.12% and the sodium hydroxide content was 32.02%.

In order to further verify, as the contact time between caustic soda and air increases, the content of sodium carbonate in caustic soda will gradually increase. On September 15th, samples were taken from the circulating alkali tank at the cathode of the electrolytic cell and left open for different periods of time for comparative analysis. The experimental data is shown in Table 3.

According to the data in Table 3, the content of sodium carbonate in caustic soda is increasing over time. This indicates that as the contact time between the alkaline solution and air increases, the sodium hydroxide in the alkaline solution reacts with the carbon dioxide in the air to form sodium carbonate, which continues to increase and reaches an extreme value of 0.39% after 72 hours.

Based on the above analysis methods and reference to literature, the quality of industrial sodium hydroxide was analyzed using the determination methods for sodium hydroxide and sodium carbonate content specified in GB/T 4348.1-2013 and the dual indicator method. Through experimental measurements, it was found that the content of sodium hydroxide and sodium carbonate in industrial sodium hydroxide analyzed by the dual indicator method differed significantly from their actual values; The content of sodium hydroxide and sodium carbonate in industrial sodium hydroxide analyzed in GB/T4348.1-2013 is consistent with the actual values. Therefore, it is recommended to use the barium chloride method specified in GB/T4348.1-2013 for analysis.

Mixed alkali is a mixture of sodium carbonate and sodium bicarbonate or sodium carbonate and sodium hydroxide, and the main method for determining mixed alkali is the dual indicator method. The color change range of the indicator is pH=pKHIn ± 1, but due to the varying sensitivity of the human eye to different colors and the mutual masking effect between the two colors, the observed color change range of the acid-base indicator may differ, leading to titration errors. By improving the sample size for measuring mixed alkali, the experimental conditions for measuring mixed alkali were explored, and the optimal conditions were found, resulting in satisfactory results.

6 Rectification measures

Through the above analysis, the key links to solve the problem have been identified. Measures such as reducing the amount of sodium bicarbonate in brine, introducing acid neutralization process during secondary refining of brine, regularly replacing ion membranes, changing the feeding method of finished alkali tanks, sealing the transportation process, adding nitrogen sealing to finished alkali tanks, and improving analysis methods have been taken to achieve the sodium carbonate content in caustic soda meeting the requirements of liquid caustic soda indicators in GB/T11199-2006.

6.1 Reduce excess sodium carbonate in the brine

In production, use the method of adding excess NaOH and Na ₂ CO3 to remove Mg ²+and Ca ²+, and the generated Mg (OH) ₂ has colloidal properties. The newly generated Mg (OH) ₂ can encapsulate the finely dispersed CaCO3 crystals and accelerate the settling rate. After sedimentation, the Ca2+and Mg2+in the saltwater decreased to below 10 mg/L.

The main purpose of adding sodium carbonate to brine is to remove calcium ions. To ensure that calcium ions in brine are completely removed, the industry generally controls the amount of sodium bicarbonate in brine at 0.25-0.40g/L. On the premise of ensuring that calcium ions can be removed, the amount of sodium carbonate added to brine can be accurately adjusted through a pure soda online detector to reduce the amount of sodium bicarbonate in brine. This not only reduces raw material consumption and saves costs, but also controls the excessive sodium carbonate in the brine entering the tank to reverse osmosis into the cathode alkali solution.

6.2 Add acid to neutralize during the secondary refining of brine

After filtration and sedimentation, the Ca²⁺and Mg²⁺content in the brine was reduced, but there were still unfiltered CaCO₃ and Mg (OH)₂. The pH value of Mg(OH)₂ particles dissolved was 10.5, and the pH value of CaCO3 dissolved was 9.4. Due to the pH value of 10.5 in brine, these particles cannot dissolve. Chelating resin can only adsorb Ca²⁺and Mg²⁺ ions, but cannot adsorb Ca and Mg components in particles. Excessive Na₂CO₃ during a brine refining process will enter the electrolytic cell along with the brine.

Before the brine enters the chelating resin tower, 31% hydrochloric acid is added and mixed evenly with the brine through a static mixer. The mixture then enters the pH automatic analyzer, where the amount of hydrochloric acid added is automatically adjusted based on the measured pH value to achieve control.

Use the acid addition process during the secondary refining of brine can dissolve the partially settled CaCO₃ and Mg(OH)₂ in the saltwater, allowing Ca²⁺and Mg²⁺ ions in the brine to be completely adsorbed when entering the chelating resin tower. Additionally, it can decompose excess Na₂CO₃ in the brine, preventing it from entering the cathode through ion membrane reverse osmosis and causing an increase in sodium carbonate content in the caustic soda solution.

6.3 Change the ion membrane

The condition of the membrane is directly related to the safety and stability of production during the operation of the electrolytic cell. When there is a leakage point in the membrane, it will present different situations at different stages of the electrolytic cell current density operation due to the size and location of the leakage point. As membrane damage intensifies, the catholyte enters the anode for reaction until it penetrates the anode disk, ultimately corroding the electrolytic cell and posing greater safety hazards. So the membrane with leakage points should be replaced as soon as possible.

6.4 Change the feeding method of the product alkali tank

By observing the feeding method of the product alkali tank area, it was found that some alkali tank feeding ports feed from the top of the tank body, and the feeding pipe does not penetrate deep into the bottom of the tank body, causing the alkali solution to come into direct contact with the air when entering the tank, absorbing carbon dioxide from the air and causing an increase in the sodium carbonate content in the caustic soda. Rectify the feeding method of the alkali tank from upper feeding to bottom feeding to prevent contact between caustic soda and carbon dioxide in the air during feeding.

6.5 Ensure proper sealing during transportation

Due to contact with air, caustic soda in tank trucks reacts with carbon dioxide in the air to form sodium carbonate, which dissolves in sodium hydroxide solution. Therefore, it is necessary to seal the caustic soda transport vehicles to prevent contact between air and caustic soda in tank trucks.

6.6 Add nitrogen seal to the product alkali tank

When loading in the product tank area, the liquid level in the alkali tank decreases, causing air to enter. The caustic soda reacts with carbon dioxide in the air to produce sodium carbonate, which dissolves in the sodium hydroxide solution. By adding a nitrogen sealing device to the existing alkali tank, it is possible to prevent the tank from inhaling air during loading and to prevent the reaction of caustic soda at the top of the tank with carbon dioxide in the air to produce sodium carbonate.

6.7 lower the temperature of product alkali

By analyzing the co-solubility of sodium hydroxide and sodium carbonate, the temperature of the finished alkaline solution can be reduced to low or normal temperature to decrease the content of sodium carbonate in the alkaline solution.

6.8 Improve analysis methods

Using the analysis method specified in GB/T 4348.1-2013, first analyze the content of sodium hydroxide in the sample solution, and then measure the total alkalinity of sodium hydroxide and sodium carbonate in the sample solution. Subtract the two to obtain the content of sodium carbonate in the sample solution. And improve the sample weight in the analysis method to reduce titration errors.

7 Conclusion

Based on the above analysis of the reasons, the quality of caustic soda products has been improved by reducing the amount of sodium bicarbonate in brine to 0.10~0.25g/L, introducing acid addition process, regularly replacing ion membranes, feeding alkaline solution from the bottom of the product alkali tank, adding nitrogen seal to the product alkali tank, sealing the alkali tank transport vehicle, and improving existing analysis methods. The sodium carbonate content in caustic soda has been significantly reduced and stabilized at 0.03%~0.05%, meeting the requirement of the high-purity sodium hydroxide quality standard (GB/T11199-2006) that the maximum sodium carbonate content cannot exceed 0.06%, and meeting the needs of users. As an experienced engineering company in the chlor-alkali field, BPC is committed to providing upgrading services for chlor-alkali clients worldwide.