Method for recovering and recycling free acid in coal ash or coal gangue acid leaching solution

Abstract

The invention belongs to the technical field of solid waste resource recycling, and particularly relates to a method for recycling and cyclic utilization of free acid in coal ash or coal gangue acid leaching liquid. The method comprises the following steps: respectively mixing fly ash or coal gangue with an alkali solution and an acid solution to obtain an acid leaching solution; then, adding a mixture of ammonium chloride, sodium chloride and EDTA (Ethylene Diamine Tetraacetic Acid) to enhance the volatility of the acid and reduce the consumption of metal ions on the acid; the mixed acid leaching solution enters a first-effect negative pressure evaporation system, and HCl + H2O mixed water vapor is obtained; then, the mixed water vapor enters a second-effect negative pressure evaporation system to be further concentrated, and finally hydrochloric acid with the concentration being 15-22.4% is obtained. According to the method, parameters of the evaporation process are accurately controlled, the acid recovery efficiency is optimized, the purity of the acid liquor is ensured, the free acid can be effectively recovered, the acid loss is reduced, the production cost is reduced through cyclic utilization, and the method has important significance on resource saving and environmental protection.

Description

Technical Field

[0001] The invention belongs to the technical field of solid waste resource recycling, and in particular relates to a method for recycling free acid in fly ash or coal gangue acid leaching solution. Background Art

[0002] my country's bauxite resources are becoming increasingly depleted, and the country's dependence on foreign bauxite is increasing year by year, which has seriously affected the sustainable development of the aluminum industry and restricted the international competitiveness of my country's aluminum industry. Against this background, finding alternative sources of bauxite resources has become one of the important issues for the development of the industry. The Ordos Basin in Inner Mongolia is rich in high-aluminum coal, and the alumina content in its fly ash after calcination can reach more than 50%. How to effectively develop and utilize resources such as aluminum and silicon in fly ash has become one of the important ways to alleviate the tension in my country's bauxite resources.

[0003] Fly ash, as an inevitable product of coal burning in thermal power plants, is one of the largest single industrial solid wastes in China. According to statistics, the total amount of fly ash produced in my country each year reaches hundreds of millions of tons. Its storage and treatment not only occupy a large amount of land resources, but also have a significant impact on the ecological environment. In addition, similar to fly ash, a large amount of coal gangue, as a by-product of coal mining, also poses a huge challenge to ecological and environmental protection. Therefore, how to realize the resource utilization of fly ash and coal gangue can not only reduce the pressure of solid waste on the environment, but also tap the potential economic value of these wastes, which has important environmental and economic benefits.

[0004] In the prior art, a method for preparing aluminum silicon oxide by using an acid-base combined treatment process of fly ash has been developed (patent number: CN116216731B). This method sequentially treats fly ash with alkali dissolution, acid leaching and roasting to finally prepare aluminum silicon oxide with high aluminum silicon content and low impurity content. This process technology efficiently separates and purifies aluminum silicon resources in fly ash, providing a feasible solution for the high value-added utilization of fly ash. However, this process inevitably produces a large amount of waste acid liquid during the acid leaching stage. These waste acid liquids contain a large amount of free acid, which will cause serious pollution to the environment if discharged directly. At the same time, the treatment of waste acid liquid also requires a lot of resources and funds, which not only increases the process cost, but also reduces the economic efficiency of resource utilization.

[0005] With the in-depth promotion of the concept of green development and the advancement of the "dual carbon" goal, it is particularly important to develop a technology that can efficiently recycle waste acid and reuse it. This can not only reduce the environmental pollution caused by waste acid emissions, but also significantly reduce the cost of acid treatment processes, and further improve the overall benefits of resource utilization. At the same time, by recycling waste acid, it can also reduce dependence on fresh acid resources, providing technical support for enterprises to achieve energy conservation and cost reduction.

[0006] At present, waste acid recovery technology is still in the development stage, mainly based on neutralization treatment, extraction separation, etc. However, these methods often have problems such as low treatment efficiency, high cost or complex operation, which are difficult to meet the needs of industrial application.

[0007] Therefore, it is urgent to develop an efficient method for recovering free acid in fly ash and coal gangue acid leaching solution to achieve the recycling of waste acid. This technology can not only provide technical support for the resource utilization of fly ash and coal gangue, but also promote the further development of the field of comprehensive utilization of solid waste and provide strong support for the realization of green industrial cycle. Summary of the invention

[0008] The purpose of the present invention is to provide a method for recovering and recycling free acid in fly ash or coal gangue acid leaching solution. The method can not only effectively recover the free acid in the acid leaching solution, but also the recovered acid can be recycled in the process of treating fly ash or coal gangue, which not only reduces pollution to the environment, but also saves costs and increases efficiency for enterprises in acid recycling.

[0009] In order to achieve the above object, the present invention provides the following technical solutions: A method for recovering and recycling free acid in fly ash or coal gangue acid leaching solution, comprising the following steps: (1) mixing fly ash or coal gangue with an alkali solution, performing an alkali dissolution reaction, filtering to obtain alkali-soluble fly ash or alkali-soluble coal gangue, mixing the alkali-soluble fly ash or alkali-soluble coal gangue with an acid solution, performing an acid dissolution reaction, filtering to obtain an acid leaching solution of fly ash or coal gangue; (2) adding a mixture of ammonium chloride, sodium chloride and EDTA to the acid leaching solution obtained in step (1) to obtain a mixed acid leaching solution, passing the mixed acid leaching solution into a single-effect negative pressure evaporation system to separate the upper layer of HCl+H2O mixed water vapor and mother liquor; (3) The separated upper layer of HCl+H2O mixed water vapor is passed into a two-effect negative pressure evaporation system for evaporation and concentration, and after cooling, dilute hydrochloric acid with a concentration of <0.5% and concentrated hydrochloric acid with a concentration of 15-22.4% are obtained; (4) The concentrated hydrochloric acid with a concentration of 15-22.4% obtained in step (3) is recycled through a circulating acid pump to the process of treating fly ash or coal gangue.

[0010] Furthermore, the alkaline solution in step (1) is a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution or an ammonia solution, the mass concentration of the alkaline solution is 2-10%, and the mass concentration ratio of fly ash or coal gangue to the alkaline solution is (10-30) g / 100 mL; Furthermore, the acid solution in step (1) is a hydrochloric acid aqueous solution with a mass concentration of 5-10%, and the mass concentration ratio of alkali-soluble fly ash or alkali-soluble coal gangue to the acid solution is (10-20) g / 100 mL.

[0011] Furthermore, the mass concentration of free acid in the acid leaching solution of step (1) is 3-15%, and the free acid comprises hydrochloric acid.

[0012] Furthermore, in step (2), the mass concentrations of ammonium chloride, sodium chloride and EDTA in the acid leaching solution are 5-15%, 5-15% and 0.5-2%, respectively.

[0013] The present invention helps to optimize the recovery of acid and improve the separation efficiency of free acid by adding a mixture of ammonium chloride, sodium chloride and EDTA to the acid leaching solution. Ammonium chloride mainly generates ammonia gas and hydrogen chloride gas by decomposition, which enhances the volatility of free acid, so that the acid can be more effectively recovered during the evaporation process. In addition, ammonium chloride also helps to improve the separation recovery rate of acid by increasing the gas concentration of hydrogen chloride. Sodium chloride is used in combination with ammonium chloride to further promote the volatility of hydrogen chloride. Sodium chloride increases the volatilization rate of hydrogen chloride through the salt effect, ensuring that more acid is separated during the evaporation process. At the same time, sodium chloride also provides chloride ions, which provides strong support for the generation of hydrogen chloride. EDTA, as a strong complexing agent, can form a stable complex with metal ions (such as iron, aluminum, magnesium, etc.) in the acid leaching solution, prevent these metal ions from reacting with free acid, reduce acid consumption, ensure that more free acid can be volatilized during the evaporation process, and avoid interference of metal ions on acid. The synergistic effect of the three enables the free acid in the acid leaching solution to be recovered more efficiently, reducing the loss of acid, while improving the efficiency of the evaporation process and the purity of the acid solution.

[0014] Furthermore, in the single-effect negative pressure evaporation system in step (2), the vacuum degree is 10-20 KPa, the evaporation temperature is 40-70° C., and the flow rate of the mixed acid leaching solution into the single-effect negative pressure evaporation system is 30-50 L / min.

[0015] The present invention can effectively reduce energy consumption and avoid the occurrence of side reactions by precisely controlling the vacuum degree and evaporation temperature of the single-effect negative pressure evaporation system. Trace metal ions (such as iron, copper, etc.) remaining in the acid leaching solution may react with acid at high temperatures to produce precipitation or oxides. By maintaining the system temperature within a relatively low range, the reactivity of these metals can be reduced, the generation of by-products can be avoided, and the purity of the free acid during the evaporation process can be ensured to be higher.

[0016] Furthermore, the mixed acid leaching solution in step (2) is first passed through a primary preheater before entering the first-effect negative pressure evaporation system. The primary preheater uses the steam condensate of the second-effect negative pressure evaporation system as a heat source and preliminarily preheats the mixed acid leaching solution to 40-50° C. through a heat exchanger.

[0017] Furthermore, the mixed acid leaching solution after the primary preheater then enters the secondary preheater, which uses the steam condensate of the single-effect negative pressure evaporation system as a heat source, and further increases the temperature of the mixed acid leaching solution to 50-60° C. through a heat exchanger.

[0018] The present invention uses an innovative design to preheat the mixed acid leaching solution through a primary preheater and a secondary preheater before passing it into a single-effect negative pressure evaporation system, effectively utilizing the waste heat of the steam condensate of the negative pressure evaporation system to heat the acid leaching solution. The preheating function of the primary preheater and the secondary preheater reduces the evaporation system's dependence on external heat sources, significantly improves the efficiency of thermal energy utilization, and reduces overall energy consumption. The primary and secondary preheating temperatures are controlled between 40-60°C to ensure that the acid leaching solution gradually reaches a temperature suitable for entering the evaporation system, thereby avoiding thermal stress or acid loss caused by direct heating to an excessively high temperature and improving the efficiency of the evaporation process.

[0019] The first-stage preheater uses the steam condensate of the two-effect negative pressure evaporation system as a heat source. In the two-effect evaporation system, the evaporated steam will be condensed and recovered to form a low-temperature condensate. Since the operating temperature of the two-effect system is high, the temperature of this part of the condensate is relatively high, so there is enough heat to heat the cold liquid. The second-stage preheater uses the steam condensate of the one-effect negative pressure evaporation system as a heat source. The one-effect evaporator is the main equipment for directly heating the acid solution. After the one-effect evaporation, the temperature of the condensate is usually low. The temperature of this part of the condensate is suitable for the second-stage preheater, and the acid solution is further heated to 50-60°C. This design of the present invention ensures that heat energy is gradually transferred from high temperature to low temperature through the principle of cascade utilization, avoiding the problem of too high temperature, overheating of the acid solution or unstable evaporation rate during the preheating process, and can use an external heating source at the same time, maximize the use of heat energy, and improve the overall utilization efficiency of energy.

[0020] Furthermore, the mother liquor in step (2) is used in the polyaluminum process or the aluminum oxide preparation process.

[0021] Furthermore, in step (3), the vacuum degree in the two-effect negative pressure evaporation system is 5-15 KPa, the evaporation temperature is 60-80°C, and the flow rate of HCl+H2O mixed water vapor entering the two-effect negative pressure evaporation system is 20-40 L / min.

[0022] The present invention can avoid the phenomenon of over-concentration in the acid solution concentration process by accurately controlling the temperature and flow rate in the two-effect negative pressure evaporation system. Over-concentration may cause excessive volatilization of free acid in the acid solution, and even cause the solution to solidify or precipitate. By accurately controlling the temperature and flow rate, the evaporation process can be ensured to be carried out gently, so that the acid solution maintains a stable state within an appropriate concentration range, thereby avoiding operational risks or system failures caused by excessive concentration. Accurately controlling the temperature and vacuum degree during the two-effect evaporation process can ensure that impurities in the acid solution do not volatilize with the steam, thereby ensuring the purity of the acid during the evaporation process.

[0023] Compared with the prior art, the advantages and beneficial effects of the present invention are: The present invention provides a method for efficiently recovering and recycling free acid in fly ash and coal gangue acid leaching solution. Through a number of innovative designs, the acid recovery rate and purity are significantly improved, while the thermal energy utilization is optimized and energy consumption is reduced. The fine design of the single-effect and double-effect negative pressure evaporation systems ensures efficient concentration and high recovery of the acid solution. At the same time, by accurately controlling the temperature and vacuum degree, the excessive concentration of the acid solution or the occurrence of side reactions is avoided, thereby improving the purity of the acid. Through the primary and secondary preheaters, the waste heat in the evaporation process is effectively recovered, the thermal energy utilization efficiency is significantly improved, and the dependence on external energy is reduced. The present invention not only improves the acid recovery rate and reduces energy consumption, but also promotes environmental protection and sustainable development, and has significant economic and environmental benefits in industrial applications. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] Example 1 This embodiment provides a method for recovering and recycling free acid in fly ash acid leaching solution, which comprises the following steps: (1) Fly ash is mixed with a 5wt% sodium hydroxide aqueous solution at a ratio of 20g / 100mL, and an alkali-soluble reaction is carried out. After filtering, an alkali-soluble fly ash is obtained. The alkali-soluble fly ash is mixed with an 8wt% hydrochloric acid aqueous solution at a ratio of 15g / 100mL, and an acid-soluble reaction is carried out. After filtering, a fly ash acid leaching solution is obtained. (2) adding a mixture of ammonium chloride, sodium chloride and EDTA to the acid leaching solution obtained in step (1) to obtain a mixed acid leaching solution, wherein the mass concentrations of ammonium chloride, sodium chloride and EDTA in the acid leaching solution are 10%, 10% and 1%, respectively; passing the mixed acid leaching solution through a primary preheater, which uses steam condensate from a two-effect negative pressure evaporation system as a heat source, and preliminarily preheating the mixed acid leaching solution to 40° C. through a heat exchanger; then, the mixed acid leaching solution enters a secondary preheater, which uses steam condensate from a single-effect negative pressure evaporation system as a heat source, and further raises the temperature of the mixed acid leaching solution to 55° C. through a heat exchanger; passing the preheated mixed acid leaching solution into a single-effect negative pressure evaporation system at a rate of 40 L / min, setting the vacuum degree to 15 KPa and the evaporation temperature to 60° C., and separating the upper layer ≤10% of HCl+H2O mixed water vapor and mother liquor; (3) The separated upper HCl+H2O mixed water vapor is passed into the two-effect negative pressure evaporation system at a flow rate of 30L / min for evaporation and concentration. The vacuum degree is set to 10KPa and the evaporation temperature is 70°C. The top flow liquid in the second-effect evaporator is HCl+H2O water vapor at the azeotropic point, which is an aqueous solution after cooling. It passes through the dilute acid cooler on the way and is cooled at the top of the tower to obtain a hydrochloric acid aqueous solution with a mass fraction of <0.5%. The cooled dilute acid enters the non-condensable gas condenser to cool and purify the non-condensable gas. After purifying the non-condensable gas, the dilute acid returns to the dilute acid tank; the bottom flow liquid is forced to circulate in the evaporation unit for multiple times and completes evaporation and concentration. After concentration, it is cooled at the bottom of the tower to obtain concentrated hydrochloric acid with a hydrogen chloride concentration of 15.03%; (4) The concentrated hydrochloric acid with a concentration of 15.03% obtained in step (3) is recycled through a circulating acid pump to the process of treating fly ash or coal gangue.

[0026] The fly ash acid leaching solution obtained in step (1) of this embodiment is 100 L, and the concentrated hydrochloric acid with a concentration of 15.03% is finally obtained in an amount of 57.3 L.

[0027] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 1.

[0028] Table 1

[0029] The recovery rate of HCl in concentrated acid was calculated to be 92.5%.

[0030] Example 2 This embodiment provides a method for recovering and recycling free acid in fly ash acid leaching solution, which comprises the following steps: (1) Fly ash is mixed with 8 wt % potassium hydroxide aqueous solution at a ratio of 20 g / 100 mL, and an alkali dissolution reaction is carried out. After filtering, an alkali-soluble fly ash is obtained. The alkali-soluble fly ash is mixed with 10 wt % hydrochloric acid aqueous solution at a ratio of 15 g / 100 mL, and an acid dissolution reaction is carried out. After filtering, an acid leaching solution of fly ash is obtained. (2) adding a mixture of ammonium chloride, sodium chloride and EDTA to the acid leaching solution obtained in step (1) to obtain a mixed acid leaching solution, wherein the mass concentrations of ammonium chloride, sodium chloride and EDTA in the acid leaching solution are 12%, 8% and 1%, respectively; passing the mixed acid leaching solution through a primary preheater, which uses steam condensate from a two-effect negative pressure evaporation system as a heat source, and preliminarily preheating the mixed acid leaching solution to 40° C. through a heat exchanger; then passing the mixed acid leaching solution into a secondary preheater, which uses steam condensate from a single-effect negative pressure evaporation system as a heat source, and further raising the temperature of the mixed acid leaching solution to 55° C. through a heat exchanger; passing the preheated mixed acid leaching solution into a single-effect negative pressure evaporation system at a rate of 40 L / min, setting the vacuum degree to 12 KPa and the evaporation temperature to 68° C., and separating the upper layer ≤10% of HCl+H2O mixed water vapor and mother liquor; (3) The separated upper HCl+H2O mixed water vapor is passed into the two-effect negative pressure evaporation system at a flow rate of 30L / min for evaporation and concentration. The vacuum degree is set to 12KPa and the evaporation temperature is 75°C. The top flow liquid in the second-effect evaporator is HCl+H2O water vapor at the azeotropic point, which is an aqueous solution after cooling. It passes through the dilute acid cooler on the way and is cooled at the top of the tower to obtain a hydrochloric acid aqueous solution with a mass fraction of <1%. The cooled dilute acid enters the non-condensable gas condenser to cool and purify the non-condensable gas. After purifying the non-condensable gas, the dilute acid returns to the dilute acid tank; the bottom flow liquid is forced to circulate multiple times in the evaporation unit and completes evaporation and concentration. After concentration, it is cooled at the bottom of the tower to obtain concentrated hydrochloric acid with a hydrogen chloride concentration of 17.54%; (4) The concentrated hydrochloric acid with a concentration of 17.54% obtained in step (3) is recycled through a circulating acid pump to the process of treating fly ash or coal gangue.

[0031] The fly ash acid leaching solution obtained in step (1) of this embodiment is 100 L, and the concentrated hydrochloric acid with a concentration of 17.54% finally obtained is 53.9 L.

[0032] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 2.

[0033] Table 2

[0034] The recovery rate of HCl in concentrated acid was calculated to be 93.2%.

[0035] Example 3 This embodiment provides a method for recovering and recycling free acid in coal gangue acid leaching solution, which comprises the following steps: (1) Mixing coal gangue with 5wt% sodium hydroxide aqueous solution at a ratio of 20g / 100mL, performing an alkali dissolution reaction, filtering to obtain alkali-soluble coal gangue, mixing the alkali-soluble coal gangue with 8wt% hydrochloric acid aqueous solution at a ratio of 15g / 100mL, performing an acid dissolution reaction, and filtering to obtain coal gangue acid leaching solution; (2) adding a mixture of ammonium chloride, sodium chloride and EDTA to the acid leaching solution obtained in step (1) to obtain a mixed acid leaching solution, wherein the mass concentrations of ammonium chloride, sodium chloride and EDTA in the acid leaching solution are 7%, 13% and 1.5%, respectively; passing the mixed acid leaching solution through a primary preheater, which uses steam condensate from a two-effect negative pressure evaporation system as a heat source, and preliminarily preheating the mixed acid leaching solution to 40° C. through a heat exchanger; then, the mixed acid leaching solution enters a secondary preheater, which uses steam condensate from a single-effect negative pressure evaporation system as a heat source, and further raises the temperature of the mixed acid leaching solution to 55° C. through a heat exchanger; passing the preheated mixed acid leaching solution into a single-effect negative pressure evaporation system at a rate of 40 L / min, setting the vacuum degree to 15 KPa and the evaporation temperature to 60° C., and separating the upper layer of HCl+H2O mixed water vapor and mother liquor of ≤10%; (3) The separated upper HCl+H2O mixed water vapor is passed into the two-effect negative pressure evaporation system at a flow rate of 30L / min for evaporation and concentration. The vacuum degree is set to 10KPa and the evaporation temperature is 70°C. The top flow liquid in the second-effect evaporator is HCl+H2O water vapor at the azeotropic point, which is an aqueous solution after cooling. It passes through the dilute acid cooler on the way and is cooled at the top of the tower to obtain a hydrochloric acid aqueous solution with a mass fraction of <2%. The cooled dilute acid enters the non-condensable gas condenser to cool and purify the non-condensable gas. After purifying the non-condensable gas, the dilute acid returns to the dilute acid tank; the bottom flow liquid is forced to circulate in the evaporation unit for multiple times and completes evaporation and concentration. After concentration, it is cooled at the bottom of the tower to obtain concentrated hydrochloric acid with a hydrogen chloride concentration of 21.69%; (4) The concentrated hydrochloric acid with a concentration of 21.69% obtained in step (3) is recycled through a circulating acid pump to the process of treating fly ash or coal gangue.

[0036] The amount of coal gangue acid leaching solution obtained in step (1) of this embodiment is 100 L, and the amount of concentrated hydrochloric acid with a concentration of 21.69% finally obtained is 45.7 L.

[0037] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 3.

[0038] Table 3

[0039] The recovery rate of HCl in concentrated acid was calculated to be 88.4%.

[0040] Comparative Example 1 This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which differs from Example 1 in that in step (2), the acid leaching solution is not added with ammonium chloride, sodium chloride and EDTA, and directly enters the primary preheater.

[0041] The fly ash acid leaching solution obtained in step (1) of this comparative example was 100 L, and 56.6 L of concentrated hydrochloric acid with a concentration of 11.30% was finally obtained.

[0042] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 4.

[0043] Table 4

[0044] The recovery rate of HCl in concentrated acid was calculated to be 75.4%.

[0045] Comparative Example 2 This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which differs from Example 1 in that ammonium chloride, sodium chloride and EDTA in step (2) are replaced by ammonium sulfate, sodium sulfate and citric acid.

[0046] The fly ash acid leaching solution obtained in step (1) of this comparative example was 100 L, and 54.5 L of concentrated hydrochloric acid with a concentration of 14.37% was finally obtained.

[0047] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 5.

[0048] Table 5

[0049] The recovery rate of HCl in concentrated acid was calculated to be 77.4%.

[0050] Comparative Example 3 This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which differs from Example 1 in that: in step (2), the preheated mixed acid leaching solution is passed into a single-effect negative pressure evaporation system at a rate of 40 L / min, the vacuum degree is set to 25 KPa, and the evaporation temperature is set to 40°C.

[0051] The fly ash acid leaching solution obtained in step (1) of this comparative example was 100 L, and 50.2 L of concentrated hydrochloric acid with a concentration of 19.26% was finally obtained.

[0052] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 6.

[0053] Table 6

[0054] The recovery rate of HCl in concentrated acid was calculated to be 81.8%.

[0055] Comparative Example 4 This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which differs from Example 1 in that in step (2), the preheated mixed acid leaching solution is passed into a single-effect negative pressure evaporation system at a rate of 60 L / min.

[0056] The fly ash acid leaching solution obtained in step (1) of this comparative example was 100 L, and 47.3 L of concentrated hydrochloric acid with a concentration of 19.23% was finally obtained.

[0057] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 7.

[0058] Table 7

[0059] The recovery rate of HCl in concentrated acid was calculated to be 83.0%.

[0060] Comparative Example 5 The present comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which differs from Example 1 in that: in step (3), the separated upper layer of HCl+H2O mixed water vapor is introduced into a two-effect negative pressure evaporation system at a flow rate of 30 L / min for evaporation and concentration, and the vacuum degree is set to 5 KPa and the evaporation temperature is set to 90°C.

[0061] The fly ash acid leaching solution obtained in step (1) of this comparative example was 100 L, and 52.8 L of concentrated hydrochloric acid with a concentration of 14.76% was finally obtained.

[0062] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 8.

[0063] Table 8

[0064] The recovery rate of HCl in concentrated acid was calculated to be 80.5%.

[0065] Comparative Example 6 This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which differs from Example 1 in that: in step (3), the separated upper layer of HCl+H2O mixed water vapor is introduced into a two-effect negative pressure evaporation system at a flow rate of 10 L / min for evaporation and concentration, and the vacuum degree is set to 20 KPa and the evaporation temperature is 50°C.

[0066] The fly ash acid leaching solution obtained in step (1) of this comparative example was 100 L, and 57.1 L of concentrated hydrochloric acid with a concentration of 13.35% was finally obtained.

[0067] The free acid and metal oxide contents at various stages of the sample treatment process are shown in Table 9.

[0068] Table 9

[0069] The recovery rate of HCl in concentrated acid was calculated to be 84.9%.

[0070] It can be seen from the above results that the method of the present invention significantly improves the recovery rate of acid, optimizes the utilization of heat energy, and reduces energy consumption. In Comparative Example 1, the mixture of ammonium chloride, sodium chloride and EDTA was not added to the acid leaching solution. The absence of these enhancers resulted in poor volatility of the acid, large acid loss during the evaporation process, and reduced recovery rate. In Comparative Example 2, ammonium chloride, sodium chloride and EDTA were replaced with ammonium sulfate, sodium sulfate and citric acid. The composition after replacement did not fully reduce the consumption of metal ions on acid, nor could it effectively complex metal ions, resulting in large acid loss. Comparative Example 3 used a lower evaporation temperature and a higher vacuum degree, the evaporation process was less efficient, and the acid volatilization was incomplete. Comparative Example 4 used a higher evaporation rate during the evaporation process, which may cause part of the acid to volatilize too quickly and be difficult to recover. Comparative Example 5 used an excessively high evaporation temperature, resulting in too fast acid volatilization and less acid recovery. The slow evaporation rate of Comparative Example 6 made the system's processing capacity insufficient, the evaporation process was relatively slow, and the acid recovery rate was relatively low.

[0071] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for recovering and recycling free acid in fly ash or coal gangue acid leaching solution, comprising the following steps: (1) mixing fly ash or coal gangue with an alkali solution, performing an alkali dissolution reaction, filtering to obtain alkali-soluble fly ash or alkali-soluble coal gangue, mixing the alkali-soluble fly ash or alkali-soluble coal gangue with an acid solution, performing an acid dissolution reaction, filtering to obtain an acid leaching solution of fly ash or coal gangue; (2) adding a mixture of ammonium chloride, sodium chloride and EDTA to the acid leaching solution obtained in step (1) to obtain a mixed acid leaching solution, passing the mixed acid leaching solution into a single-effect negative pressure evaporation system to separate the upper layer of HCl+H2O mixed water vapor and mother liquor; (3) The separated upper layer of HCl+H2O mixed water vapor is passed into a two-effect negative pressure evaporation system for evaporation and concentration, and after cooling, dilute hydrochloric acid with a concentration of <0.5% and concentrated hydrochloric acid with a concentration of 15-22.4% are obtained; (4) The concentrated hydrochloric acid with a concentration of 15-22.4% obtained in step (3) is recycled through a circulating acid pump to the process of treating fly ash or coal gangue.

2. The method according to claim 1, characterized in that The alkaline solution in step (1) is a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution or an ammonia solution, the mass concentration of the alkaline solution is 2-10%, and the mass concentration ratio of fly ash or coal gangue to the alkaline solution is (10-30) g / 100 mL.

3. The method according to claim 1, characterized in that: The acid solution in step (1) is a hydrochloric acid aqueous solution with a mass concentration of 5-10%, and the mass concentration ratio of alkali-soluble fly ash or alkali-soluble coal gangue to the acid solution is (10-20) g / 100 mL.

4. The method according to claim 1, characterized in that: The mass concentration of free acid in the acid leaching solution of step (1) is 3-15%, and the free acid comprises hydrochloric acid.

5. The method according to claim 1, characterized in that In step (2), the mass concentrations of ammonium chloride, sodium chloride and EDTA in the acid leaching solution are 5-15%, 5-15% and 0.5-2% respectively.

6. The method according to claim 1, characterized in that The vacuum degree in the single-effect negative pressure evaporation system in step (2) is 10-20 KPa, the evaporation temperature is 40-70° C., and the flow rate of the mixed acid leaching solution into the single-effect negative pressure evaporation system is 30-50 L / min.

7. The method according to claim 1, characterized in that In step (2), the mixed acid leaching solution passes through a primary preheater before entering the first-effect negative pressure evaporation system. The primary preheater uses the steam condensate of the second-effect negative pressure evaporation system as a heat source and preliminarily preheats the mixed acid leaching solution to 40-50° C. through a heat exchanger.

8. The method according to claim 7, characterized in that The mixed acid leaching solution after the primary preheater then enters the secondary preheater, which uses the steam condensate of the single-effect negative pressure evaporation system as a heat source and further increases the temperature of the mixed acid leaching solution to 50-60°C through a heat exchanger.

9. The method according to claim 1, characterized in that: The mother liquor in step (2) is used in the polyaluminum process or the aluminum oxide preparation process.

10. The method according to claim 1, characterized in that The vacuum degree in the two-effect negative pressure evaporation system in step (3) is 5-15 KPa, the evaporation temperature is 60-80°C, and the flow rate of HCl+H2O mixed water vapor entering the two-effect negative pressure evaporation system is 20-40 L / min.

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