At present, sodium hypochlorite solution produced from salt and water is widely used. For example, BLUEWAV high-concentration series sodium hypochlorite generator, the generated dangerous gases chlorine and hydrogen are discharged after treatment. Can we use different forms of these two gases to produce more sodium hypochlorite solution?
Sodium hypochlorite solution was prepared by mixing chlorine condensate, hydrogen condensate and tap water with caustic soda solution to prepare absorption solution, and the effective chlorine content and stability of sodium hypochlorite solution were compared. The results show that using chlorine condensate or hydrogen condensate to produce sodium hypochlorite can not only reduce the production cost but also improve the product stability.
Chlorine condensate
In the production of chlor-alkali, the chlorine gas produced by electrolysis has a high moisture content and is highly corrosive, which is not easy to transport and use. The moisture in the wet chlorine gas is generally removed by cooling and cooling – sulfuric acid absorption to meet the requirements of the downstream production of chlorine products. Most of the water is separated after washing heat transfer and indirect heat transfer in the chlorine hydrogen treatment process. The water separated from this part is traditionally called chlorine water (the following chlorine condensate water is called chlorine water). There is a large amount of chlorine gas dissolved in chlorine water, which exists in the form of harmful substances in the production. The free chlorine dissolved in chlorine water corrode carbon steel equipment and destroy chelating resin Process. The typical process of chlorinated water treatment in ionic membrane production process is to transport chlorinated water to the electric chlorine release unit, heat, add acid to adjust pH value, vacuum dechlorination, chemical dechlorination, and then prepare brine. Although this solves the problem of direct emissions polluting the environment, the treatment costs brought by this are also rising.
Chlorinated water dechlorination treatment and material consumption
The dechlorination process is as follows: the temperature of chlorine water is raised to about 85℃ with steam, and 32%(mass fraction, the same below) hydrochloric acid is added to adjust the pH value to about 1.3, and the vacuum degree is 65 kPa under the condition of physical dechlorination; After most of the free chlorine is removed, 30% caustic soda is added to the chlorine water to adjust the pH value of the solution to 9~11, and then 9% sodium sulfite solution is added to further eliminate the residual free chlorine, control the excess sodium sulfite, and remove free chlorine when the detection value ORP is less than -50mV. Therefore, if the chlorine water is recycled into the system for use, it must be processed by heating, adding acid, vacuuming, adding alkali, adding sodium sulfite and other materials before it can be used for production, and the processing cost is high.
Hydrogen condensate
The saturated steam in the hydrogen gas condenses and exists in the form of condensate water, which is also used to make salt water.
Study on sodium hypochlorite and its stability
Sodium hypochlorite is widely used as a strong oxidant, bleach, disinfectant and deodorant.
In the production of chlor-alkali, sodium hypochlorite is generally present as a by-product of the treatment of chlorine gas compression process, liquid chlorine packaging process, and the exhaust gas of the plant. Due to the instability of hypochlorite, sodium hypochlorite usually has a short storage time. Many scholars have studied and discussed the decomposition characteristics of sodium hypochlorite and the methods to improve its stability. It is generally believed that temperature, light, pH value, metal ions are more important influencing factors, and one reason for adding stabilizers is to eliminate the influence of metal ions.
In theory, chlorine water and hydrogen condensate are free of heavy metal ions. Therefore, if chlorine water or hydrogen condensate is mixed with caustic soda solution to prepare an absorption liquid to absorb waste chlorine gas, the stability of sodium hypochlorite solution can be improved from the aspects of metal ions and NaCl content, and the quality of sodium hypochlorite can be improved. At the same time, the cost of dechlorination, that is, the treatment cost, will also decrease.
Determination of impurities in chlorine and hydrogen condensate
Analytical instruments: electronic balance (AUY220), 250mL volumetric bottle, 250m iodine volumetric bottle, 50m basic burette, 25mL pipette, 10m pipette, ICP analyzer (Agilent), volumetric bottle, automatic liquid gun, 1+5 hydrogen peroxide solution.
Reagents: sodium thiosulfate solution, potassium iodide solution, starch indicator, acetic acid solution, 3% sulfuric acid, arsenic standard solution, lead standard solution. Take a certain amount of chlorine water and divide it into two equal parts. The effective chlorine content in chlorine water was determined according to GB19106-2013. After sodium sulfite reagent was added to the other part to eliminate the effective chlorine in the chlorine water, the content of Mn, Pb, Fe and other heavy metal ions was measured by ICP analyzer. Measurement of chloride ions in chlorine water according to GB13025.5-2012. The heavy metal ion content of hydrogen condensate was measured by ICP analyzer, and the alkali content was measured according to GB/T 4348.1-2013.
It can be seen from the test results that chlorine water and hydrogen condensate water are better than tap water in terms of heavy metal ion content. Therefore, the alkali solution prepared with chlorine water and hydrogen condensate water should be better than that prepared with tap water to absorb chlorine gas; In addition, the material consumption of chlorinated water dechlorination is saved.
Condensate production of sodium hypochlorite test
Sodium hypochlorite was prepared by using recycled lye of similar concentration prepared with chlorine water, hydrogen condensate water and tap water respectively. By comparing the alkali consumption and the quality of sodium hypochlorite, the preferred scheme for preparing recycled lye was obtained. A certain volume of chlorine water/hydrogen condensate water/tap water was mixed with 32% caustic soda in a certain proportion, and the alkali concentration of the solution was analyzed.
Measure 440m caustic soda solution, put it into a conical bottle, put the conical bottle into flowing cold water (cold water temperature 15℃) and slowly inject chlorine gas at a rate of 15L/h, and shake the conical bottle to ensure that chlorine gas does not escape, and analyze the concentration of effective chlorine and alkali in the solution after 100min of chlorine. Pour the reaction solution into a clean brown reagent bottle and place it in the workshop. The workshop should be naturally ventilated. There is no direct sunlight in the reagent bottle, but there is scattered light. The mouth of the bottle is open, and there is a shield at the upper part of about 5cm to prevent foreign bodies from falling directly into the bottle. After 15 days of stirring in the plant, the effective chlorine content of the solution in the reagent bottle was analyzed. There is little difference in the content of available chlorine in the products, which can be ignored in industrial production. However, the use of chlorine water or hydrogen condensate and caustic soda to prepare the absorption solution for the production or by-production of sodium hypochlorite, the stability of the product is better than that of tap water and caustic soda. Using condensate water to prepare absorption liquid for chlorine gas absorption can save the steam, caustic soda, hydrochloric acid consumed by chlorine water dechlorination, so as to achieve the purpose of energy saving and consumption reduction, and in the process of using brine to make alkali, it can also increase the use of brine.
Matters needing attention
When preparing the absorption solution with chlorine water, the heat released by the reaction of chlorine and alkali should be taken into account, and the temperature of chlorine water should be controlled when mixing to prevent the volatilization of free chlorine.
Reprinted from Chlor-Alkali Industry, Volume 60, Issue 9, September 2024
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