JPH032031B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to adsorption separation of chromium ions in an aqueous solution using activated carbon. (Prior art) The presence of hexavalent chromium in wastewater and other aqueous solutions is an extremely harmful substance in relation to the human living environment, and there have been various methods for reducing it to trivalent to render it harmless or separating it. It has been proposed. In these treatment methods, even in the case of reduction to trivalent chromium ions, the final step is to remove or recover chromium ions, and these treatment methods include precipitation separation using chemicals, electrolysis, ion exchange resin, activated carbon, etc. Various adsorption methods have been proposed. Among these methods, the adsorption method using activated carbon is often used because the equipment is simple, the processing cost is low, and the adsorption rate is relatively good.
Particularly when obtaining useful substances from the mother liquor after treatment, this method has the advantage of not only removing chromium but also purifying the mother liquor by decolorizing, deodorizing, etc. (Problems to be Solved by the Invention) However, in the adsorption of chromium ions by activated carbon, the liquid properties of the solution containing chromium ions have conventionally been required to be in the acidic range, and in the alkaline range. , the adsorption efficiency is extremely reduced,
It was said that it was not possible to effectively adsorb chromium ions, and that there was almost no adsorption, especially at pH values of 10 or higher (Journal of the Chemical Society of Japan 1977, No. 3 P. 387)
~390, Water and Wastewater Vol21, No.12, (1979) P.64~
72, Japanese Patent Publication No. 52-150316). Therefore, if the treatment solution containing chromium ions is alkaline, it is necessary to adjust the solution to the optimal acidic range by acid neutralization, etc. However, in order to recover the active ingredients from the mother liquor after the removal of chromium ions Activated carbon adsorption cannot be used in cases where it is desired to process the liquid while keeping it alkaline, or where the active ingredients in the mother liquor are affected by acid. (Means for Solving the Problems) The present inventor has conducted extensive research in order to solve the above-mentioned problems, and has completed the present invention. That is, the method for adsorbing and removing chromium ions of the present invention is characterized by bringing an alkaline aqueous solution containing chromium ions into contact with gas-activated activated carbon. The activated carbon used in the method of the present invention is activated carbon activated by a gas activation method in the presence of oxygen and water vapor, and by employing activated carbon activated by this method, it can be used even in a highly alkaline region with a pH of 10 or higher. It has become possible to adsorb and remove chromium ions. On the other hand, activated carbon treated with an acid, especially a chemical, as an activation method is unsuitable because it hardly adsorbs chromium ions in the alkaline region.
Chromium can be easily removed from activated carbon that has adsorbed chromium by washing it with a diluted acid, for example, a dilute sulfuric acid solution of about 10%.After this, after washing with neutralized water, the activated carbon can be washed with an oxygen concentration of 0.5 to 1.0%. temperature in water vapor
By heating at 900 to 1000°C for about 10 minutes, it can be reactivated to the same adsorption power as fresh charcoal. Even if this reactivation is repeated at least five times, the chromium adsorption ability of the activated carbon remains almost unchanged from the initial state. The adsorption rate of chromium on activated carbon is relatively slow150
Since a contact time of ~200 hours is required, a batch method in which activated carbon is stirred and separated into a solution containing chromium ions, or a method in which a chromium ion-containing solution is circulated through a packed bed of activated carbon, is sufficient. Adsorption separation can be performed. The adsorption temperature may be at room temperature or higher, but the higher the temperature, the greater the adsorption capacity.For example, when the temperature is 50°C compared to 20°C, the adsorption capacity is about twice as high. Therefore, it is desirable that the temperature be as high as possible in view of the heat resistance of the substance to be purified in the solution and handling of the solution. (Function) The method for adsorption and removal of chromium of the present invention utilizes the fact that activated carbon activated by a gas activation method has the ability to adsorb chromium ions even in an alkaline region with a pH of 10 or higher. The adsorption rate is considerably lower than in the acidic region, but the contact time
By using the method of 200 hours, it is possible to reduce the chromium concentration to a sufficiently low adsorption equilibrium concentration. (Example) Hereinafter, the present invention will be specifically explained with reference to Examples. Example 1 In a urea synthesis process in which urea is obtained by reacting ammonia and carbon dioxide under high temperature and high pressure, the urea separation mother liquor obtained by crystallizing and separating urea from the synthesis solution is subjected to ion exchange treatment to obtain a guanidine carbonate eluent. By concentrating this eluate, the guanidine carbonate separation mother liquor in the guanidine carbonate recovery process in which guanidine carbonate is crystallized and separated is added to the next guanidine carbonate eluent and recycled. Therefore, trace amounts of chromium ions due to corrosion of the SUS material, which is the material for the urea synthesis equipment, are gradually concentrated and contained in the guanidine carbonate separation mother liquor. The guanidine carbonate crystals obtained from such a separated mother liquor come to contain chromium, particularly hexavalent chromium. In this example, the method of the present invention was carried out on a guanidine carbonate separation mother liquor containing such chromium ions. That is, a test for adsorption and removal of chromium ions was conducted by circulating the separated mother liquor of guanidine carbonate through an activated carbon packed bed using the apparatus shown in FIG. The test conditions were as follows: activated carbon brand: Toyo Calgon CAL (gas activated carbon), activated carbon filling amount: 200g, treated mother liquor amount: 4l, adsorption temperature: 50℃, SV: 5l/Hr・l (activated carbon), and the flow sheet is shown in Figure 1. The test was conducted using the apparatus as follows. After a stock solution containing chromium ions is stored in a mother liquor tank 2 through a line 1, it is pumped up by a pump 6 and sent to an adsorption tower 3. The adsorption tower 3 is heated to a predetermined adsorption temperature by passing hot water through the jacket 5. The mother liquor that has passed through the activated carbon packed bed 4 of the adsorption tower 3 is returned to the mother liquor tank 2 via the circulation line 7 and is again circulated to the adsorption tower 3 by the pump 6.The mother liquor, which has been subjected to circulation adsorption for a predetermined period of time and has adsorbed and removed chromium ions, is returned to the mother liquor tank 2 via the circulation line 7. Discharge outside the system. In addition, in this test device, continuous processing can be performed while maintaining a predetermined adsorption time by continuously controlling the intake of the stock solution and the discharge of the mother liquor. Table 1 shows the results of testing the relationship between chromium adsorption and pH of the liquid, and Table 2 shows the results of repeatedly testing the adsorption capacity of reactivated carbon.

【table】

[Table] Example 2 Figure 2 shows the results of testing the ability of various activated carbons to adsorb and remove chromium ions on fresh carbon using the adsorption test apparatus of Example 1 using a one-pass flow method without circulating the mother liquor. Test conditions Activated carbon filling amount: 200g Adsorption temperature: 50℃ SV: 5l/Hr・l (activated carbon) PH: 10 Type of test activated carbon and chromium ion concentration of supplied stock solution

[Table] However, the reason why the chromium ion concentrations of the raw solutions No. 4, 5, and 6 are lower than those of No. 1, 2, and 3 is because it was predicted in preliminary tests that the adsorption capacity was low, so the concentration was adjusted to an easily measurable concentration in advance. It has been adjusted. In addition, in Figure 2, the adsorption rate (%) is as follows: Adsorption rate (%) = (Chromium ion concentration in the stock solution supplied) - (Chromium ion concentration in the mother liquor at the adsorption tower outlet) / (Chromium ion concentration in the stock solution supplied) x 100 (Effect of the invention) The method of the present invention allows relatively easy adsorption and removal of chromium ions from solutions in the alkaline region, which has been considered difficult in the past, and is extremely effective especially for purifying alkaline product solutions to reduce hexavalent chromium, where the liquid properties cannot be changed. be. Furthermore, activated carbon as an adsorbent can be easily reactivated repeatedly and can be used repeatedly five or more times, so it is economically viable.

[Brief explanation of drawings]

FIG. 1 is a flow diagram showing the adsorption apparatus used in the adsorption and removal test of chromium ions in Example 1. In the figure, each symbol is as follows. 1... Raw solution receiving line, 2... Mother liquor tank, 3
... Adsorption tower, 4 ... Activated carbon packed bed, 5 ... Jacket, 6 ... Pump, 7 ... Circulation line, 8 ...
discharge line. FIG. 2 is a graph showing the difference in adsorption capacity of various activated carbons in Example 2. In the figure,
Each code is as follows. 11...Gas-activated carbon with oxygen-containing steam, 1
2... Inert gas heat activated bone charcoal, 13... Acid treated activated carbon, 14... Chemical treated activated carbon (1), 15... Chemical treated activated carbon (2), 16... Inert gas heat activated carbon.

Claims (1)

[Claims] 1. A method for adsorption and removal of chromium ions, which comprises bringing an alkaline aqueous solution containing chromium ions into contact with gas-activated activated carbon. 2. The method according to claim 1, wherein an alkaline aqueous solution containing chromium ions is circulated through and brought into contact with a packed bed of activated carbon. 3. The method according to claim 1, wherein the gas-activated activated carbon is activated carbon activated by a gas activation method in which the activated carbon is activated in the presence of oxygen and water vapor. 4 The pH of an alkaline aqueous solution containing chromium ions is
10. The method according to claim 1, wherein the number is in the range of 10 to 12.