A method for recovering and recycling free acid in acid leaching solution of fly ash or coal gangue

Abstract

The present invention belongs to the technical field of solid waste resource recovery and utilization, and particularly relates to a method for recovering and recycling free acid in acid leaching solution of fly ash or coal gangue. The method includes: mixing fly ash or coal gangue with an alkali solution and an acid solution respectively to obtain an acid leaching solution; then, adding a mixture of ammonium chloride, sodium chloride and EDTA to enhance the volatility of the acid and reduce the consumption of the acid by metal ions; the mixed acid leaching solution enters a first-effect negative-pressure evaporation system to obtain HCl + H2O mixed water vapor; then, the mixed water vapor enters a second-effect negative-pressure evaporation system for further concentration, and finally hydrochloric acid with a concentration of 15-22.4% is obtained. The present invention optimizes the acid recovery efficiency and ensures the purity of the acid solution by precisely controlling the parameters of the evaporation process. This method can effectively recover free acid, reduce acid loss, and reduce production costs through recycling, which is of great significance for resource conservation and environmental protection.

Description

Technical Field

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

[0002] In the prior art, an acid-base combined treatment process for using fly ash to prepare aluminum-silicon oxide has been developed (Patent No.: CN116216731B). This method sequentially performs alkali leaching treatment, acid leaching treatment, and roasting treatment on fly ash, and finally prepares aluminum-silicon oxide with high aluminum and silicon content and low impurity content. This process technology efficiently separates and purifies aluminum and silicon resources in fly ash, providing a feasible solution for the high-value utilization of fly ash. However, a large amount of waste acid solution is inevitably generated in the acid leaching stage of this process. These waste acid solutions contain a large amount of free acid, which will cause serious pollution to the environment if directly discharged. At the same time, the treatment of waste acid solution also requires a large amount of resources and funds, which not only increases the process cost but also reduces the economic efficiency of resource utilization.

[0003] Currently, 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, and are difficult to meet the requirements of industrial applications.

[0004] Therefore, there is an urgent need to develop an efficient recovery method for free acid in acid leaching solution of fly ash and coal gangue 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 realizing green industrial cycles. Summary of the Invention

[0005] The object of the present invention is to provide a method for recovering and recycling free acid in acid leaching solution of fly ash or coal gangue. The method can not only effectively recover the free acid in the acid leaching solution, but also the recovered acid can be recycled for the process of treating fly ash or coal gangue, which not only reduces environmental pollution but also saves costs and increases efficiency for enterprises in acid recycling.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for recovering and recycling free acid in acid leaching solution of fly ash or coal gangue, which comprises the following steps:

[0008] (1) Mix fly ash or coal gangue with an alkali solution for an alkali dissolution reaction. After filtration, alkali-dissolved fly ash or alkali-dissolved coal gangue is obtained. Mix the alkali-dissolved fly ash or alkali-dissolved coal gangue with an acid solution for an acid dissolution reaction. After filtration, an acid leaching solution of fly ash or coal gangue is obtained.

[0009] (2) Add 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. Pass the mixed acid leaching solution into a first-effect negative pressure evaporation system to separate the upper-layer HCl + H2O mixed water vapor and the mother liquor.

[0010] (3) Pass the separated upper-layer HCl + H2O mixed water vapor into a second-effect negative pressure evaporation system for evaporation and concentration. After cooling, dilute hydrochloric acid with a concentration < 0.5% and concentrated hydrochloric acid with a concentration of 15 - 22.4% are obtained.

[0011] (4) The concentrated hydrochloric acid with a concentration of 15 - 22.4% obtained in step (3) is recycled to the process of treating fly ash or coal gangue through a circulating acid pump.

[0012] Further, the alkali solution in step (1) is an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, or ammonia water. The mass concentration of the alkali solution is 2 - 10%, and the mass concentration ratio of fly ash or coal gangue to the alkali solution is (10 - 30) g / 100 mL.

[0013] Further, the acid solution in step (1) is an aqueous hydrochloric acid solution with a mass concentration of 5 - 10%, and the mass concentration ratio of alkali-dissolved fly ash or alkali-dissolved coal gangue to the acid solution is (10 - 20) g / 100 mL.

[0014] Further, the mass concentration of the free acid in the acid leaching solution in step (1) is 3 - 15%, and the free acid contains hydrochloric acid.

[0015] Further, the mass concentrations of ammonium chloride, sodium chloride, and EDTA in the acid leaching solution in step (2) are 5 - 15%, 5 - 15%, and 0.5 - 2% respectively.

[0016] 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 decomposes to generate ammonia and hydrogen chloride gas, enhancing the volatility of free acid, so that the acid can be more effectively recovered during evaporation. In addition, ammonium chloride also helps to improve the separation and 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 evaporation rate of hydrogen chloride through the salt effect, ensuring that more acid is separated during evaporation. At the same time, sodium chloride also provides chloride ions, providing strong support for the generation of hydrogen chloride. As a strong complexing agent, EDTA can form stable complexes with metal ions (such as iron, aluminum, magnesium, etc.) in the acid leaching solution, preventing these metal ions from reacting with free acid, reducing the consumption of acid, ensuring that more free acid can volatilize during evaporation, and avoiding the interference of metal ions on the 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.

[0017] Further, in the first-effect negative-pressure evaporation system described 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 first-effect negative-pressure evaporation system is 30 - 50 L / min.

[0018] By precisely controlling the vacuum degree and evaporation temperature of the first-effect negative-pressure evaporation system, the present invention can not only effectively reduce energy consumption, but also effectively avoid the occurrence of side reactions. Trace metal ions (such as iron, copper, etc.) remaining in the acid leaching solution may react with the acid at high temperatures to produce precipitates or oxides. By maintaining the system temperature within a lower 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 evaporation can be ensured to be higher.

[0019] Further, before the mixed acid leaching solution described in step (2) enters the first-effect negative-pressure evaporation system, it first passes through a primary preheater. 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.

[0020] Further, the mixed acid leaching solution passing through the primary preheater then enters a secondary preheater. The secondary preheater uses the steam condensate of the first-effect negative-pressure evaporation system as a heat source, and further raises the temperature of the mixed acid leaching solution to 50 - 60 °C through a heat exchanger.

[0021] Through innovative design, before the mixed acid leaching solution enters the first-effect negative-pressure evaporation system, it first passes through a primary preheater and a secondary preheater for preheating, effectively utilizing the waste heat of the steam condensate in the negative-pressure evaporation system to heat the acid leaching solution. The preheating functions of the primary preheater and the secondary preheater reduce the dependence of the evaporation system on external heat sources, significantly improve the thermal energy utilization efficiency, and reduce the overall energy consumption. The temperatures of the primary and secondary preheating are controlled between 40 - 60 °C, ensuring that the acid leaching solution gradually reaches a temperature suitable for entering the evaporation system, which can avoid thermal stress or acid loss caused by direct heating to too high a temperature and improve the efficiency of the evaporation process.

[0022] The primary preheater uses the steam condensate of the second-effect negative-pressure evaporation system as the heat source. In the second-effect evaporation system, the evaporated steam will be condensed and recovered to form low-temperature condensate. Due to the relatively high operating temperature of the second-effect system, the temperature of this part of the condensate is relatively high, so it has sufficient heat to heat the cold liquid. The secondary preheater uses the steam condensate of the first-effect negative-pressure evaporation system as the heat source. The first-effect evaporator is the main equipment for directly heating the acid solution. After the first-effect evaporation, the temperature of the condensate is usually low, and the temperature of this part of the condensate is suitable for the secondary preheater to further heat the acid solution to 50 - 60 °C. This design of the present invention ensures the gradual transfer of thermal energy from high temperature to low temperature through the principle of cascade utilization, avoiding problems such as excessive heating of the acid solution, unstable evaporation rate during the preheating process, and at the same time being able to use external heat sources to maximize the utilization of thermal energy and improve the overall energy utilization efficiency.

[0023] Furthermore, the mother liquor described in step (2) is used in the polyaluminum process or the alumina preparation process.

[0024] Furthermore, in the second-effect negative-pressure evaporation system described in step (3), the vacuum degree is 5 - 15 KPa, the evaporation temperature is 60 - 80 °C, and the flow rate of the HCl + H2O mixed water vapor entering the second-effect negative-pressure evaporation system is 20 - 40 L / min.

[0025] In the second-effect negative-pressure evaporation system of the present invention, by precisely controlling the temperature and flow rate, the phenomenon of over-concentration during the acid solution concentration process can be avoided. Over-concentration may cause excessive volatilization of free acid in the acid solution, and even lead to solidification of the solution or precipitation. By precisely controlling the temperature and flow rate, the evaporation process can be ensured to proceed gently, enabling the acid solution to maintain a stable state within an appropriate concentration range, thereby avoiding operation risks or system failures caused by too high a concentration. Precisely controlling the temperature and vacuum degree during the second-effect evaporation can ensure that impurities in the acid solution do not volatilize with the steam, guaranteeing the purity of the acid during the evaporation process.

[0026] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0027] The present invention provides a method for efficiently recovering and recycling free acid in the acid leaching solution of fly ash and coal gangue. Through a number of innovative designs, the acid recovery rate and purity are significantly improved. At the same time, the thermal energy utilization is optimized and the energy consumption is reduced. The refined design of the first-effect and second-effect negative-pressure evaporation systems ensures the efficient concentration and high recovery rate of the acid solution. At the same time, by precisely controlling the temperature and vacuum degree, the over-concentration of the acid solution or the occurrence of side reactions are avoided, and the purity of the acid is improved. Through the first-stage and second-stage 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, reduces the energy consumption, but also promotes environmental protection and sustainable development, and has significant economic and environmental benefits in industrial applications. Detailed implementation manners

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Apparently, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0029] Example 1

[0030] This embodiment provides a method for recovering and recycling free acid in the acid leaching solution of fly ash, which includes the following steps:

[0031] (1) Mix fly ash with a 5wt% sodium hydroxide aqueous solution at a ratio of 20g / 100mL for an alkali dissolution reaction. After filtration, the alkali-dissolved fly ash is obtained. The alkali-dissolved fly ash is mixed with an 8wt% hydrochloric acid aqueous solution at a ratio of 15g / 100mL for an acid dissolution reaction. After filtration, the fly ash acid leaching solution is obtained;

[0032] (2) Add 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. Among them, the mass concentrations of ammonium chloride, sodium chloride and EDTA in the acid leaching solution are 10%, 10% and 1% respectively. The mixed acid leaching solution passes through a first-stage preheater. The first-stage preheater uses the steam condensate of the second-effect negative-pressure evaporation system as the heat source, and the mixed acid leaching solution is preliminarily preheated to 40°C through a heat exchanger. Subsequently, the mixed acid leaching solution enters the second-stage preheater. The second-stage preheater uses the steam condensate of the first-effect negative-pressure evaporation system as the heat source, and the temperature of the mixed acid leaching solution is further increased to 55°C through a heat exchanger. The preheated mixed acid leaching solution is introduced into the first-effect negative-pressure evaporation system at a speed of 40L / min, the vacuum degree is set to 15KPa, and the evaporation temperature is 60°C. The upper-layer HCl + H2O mixed water vapor with a content of ≤10% and the mother liquor are separated;

[0033] (3) Feed the separated upper-layer HCl + H₂O mixed water vapor into a double-effect negative-pressure evaporation system at a flow rate of 30 L / min for evaporation and concentration. Set the vacuum degree to 10 KPa and the evaporation temperature to 70 °C. The top-flow liquid in the double-effect evaporator is HCl + H₂O water vapor at the azeotropic point, which becomes an aqueous solution after cooling. It passes through a dilute acid cooler on the way, and hydrochloric acid aqueous solution with a mass fraction <0.5% is obtained after cooling at the top of the tower. The cooled dilute acid enters a 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 undergoes multiple forced circulations in the evaporation unit and completes evaporation and concentration. After cooling at the bottom of the tower after concentration, concentrated hydrochloric acid with a hydrogen chloride concentration of 15.03% is obtained;

[0034] (4) The concentrated hydrochloric acid with a concentration of 15.03% obtained in step (3) is recycled to the process of treating fly ash or coal gangue through a circulating acid pump.

[0035] The fly ash acid leaching solution obtained in step (1) of this example is 100 L, and the finally obtained concentrated hydrochloric acid with a concentration of 15.03% is 57.3 L.

[0036] The content of free acid and metal oxides at each stage during the sample treatment process is shown in Table 1.

[0037] Table 1

[0038]

[0039] After calculation, the recovery rate of HCl in the concentrated acid is 92.5%.

[0040] Example 2

[0041] This example provides a method for recovering and recycling free acid in fly ash acid leaching solution, which includes the following steps:

[0042] (1) Mix fly ash with 8 wt% potassium hydroxide aqueous solution at a ratio of 20 g / 100 mL for an alkali dissolution reaction. After filtration, alkali-dissolved fly ash is obtained. Mix the alkali-dissolved fly ash with 10 wt% hydrochloric acid aqueous solution at a ratio of 15 g / 100 mL for an acid dissolution reaction. After filtration, fly ash acid leaching solution is obtained;

[0043] (2) Add 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. Among them, the mass concentrations of ammonium chloride, sodium chloride, and EDTA in the acid leaching solution are 12%, 8%, and 1% respectively. Pass the mixed acid leaching solution through a primary preheater. The primary preheater uses the steam condensate of a two-effect negative pressure evaporation system as the heat source. Through a heat exchanger, the mixed acid leaching solution is preliminarily preheated to 40°C. Subsequently, the mixed acid leaching solution enters a secondary preheater. The secondary preheater uses the steam condensate of a single-effect negative pressure evaporation system as the heat source. Through a heat exchanger, the temperature of the mixed acid leaching solution is further increased to 55°C. Pass the preheated mixed acid leaching solution into a single-effect negative pressure evaporation system at a speed of 40 L / min. Set the vacuum degree to 12 KPa and the evaporation temperature to 68°C to separate the upper layer of HCl + H2O mixed water vapor with a content of ≤10% and the mother liquor;

[0044] (3) Pass the separated upper layer of HCl + H2O mixed water vapor into a two-effect negative pressure evaporation system for evaporation and concentration at a flow rate of 30 L / min. Set the vacuum degree to 12 KPa and the evaporation temperature to 75°C. The top-flow liquid in the two-effect evaporator is HCl + H2O water vapor at the azeotropic point. After cooling, it is an aqueous solution. It passes through a dilute acid cooler on the way. The hydrochloric acid aqueous solution with a mass fraction of <1% is obtained by cooling at the top of the tower. The cooled dilute acid enters a non-condensable gas condenser to cool and purify the non-condensable gas. The dilute acid after purifying the non-condensable gas returns to the dilute acid tank; The bottom-flow liquid undergoes multiple forced circulations in the evaporation unit and completes evaporation and concentration. After cooling at the bottom of the tower after concentration, concentrated hydrochloric acid with a hydrogen chloride concentration of 17.54% is obtained;

[0045] (4) The concentrated hydrochloric acid with a concentration of 17.54% obtained in step (3) is recycled to the process of treating fly ash or coal gangue through a circulating acid pump.

[0046] In this example, the fly ash acid leaching solution obtained in step (1) is 100 L, and the finally obtained concentrated hydrochloric acid with a concentration of 17.54% is 53.9 L.

[0047] The content of free acid and metal oxides at each stage during the sample treatment process is shown in Table 2.

[0048] Table 2

[0049]

[0050] After calculation, the recovery rate of HCl in the concentrated acid is 93.2%.

[0051] Example 3

[0052] This example provides a method for recovering and recycling free acid in a coal gangue acid leaching solution, which includes the following steps:

[0053] (1)Mix gangue with 5 wt% aqueous sodium hydroxide solution at a ratio of 20 g / 100 mL for an alkali dissolution reaction. After filtration, the alkali-dissolved gangue is obtained. Then mix the alkali-dissolved gangue with 8 wt% aqueous hydrochloric acid solution at a ratio of 15 g / 100 mL for an acid dissolution reaction. After filtration, the acid leaching solution of gangue is obtained;

[0054] (2)Add 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. Among them, the mass concentrations of ammonium chloride, sodium chloride, and EDTA in the acid leaching solution are 7%, 13%, and 1.5% respectively. Pass the mixed acid leaching solution through a primary preheater. The primary preheater uses the steam condensate of a two-effect negative pressure evaporation system as a heat source, and preliminarily preheats the mixed acid leaching solution to 40 °C through a heat exchanger. Subsequently, the mixed acid leaching solution enters a secondary preheater. The secondary preheater uses the steam condensate of 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. Pass the preheated mixed acid leaching solution into a single-effect negative pressure evaporation system at a speed of 40 L / min, set the vacuum degree to 15 KPa, and the evaporation temperature to 60 °C to separate the upper layer of HCl + H2O mixed water vapor with a content of ≤ 10% and the mother liquor;

[0055] (3)Pass the separated upper layer of HCl + H2O mixed water vapor into a two-effect negative pressure evaporation system for evaporation and concentration at a flow rate of 30 L / min. Set the vacuum degree to 10 KPa and the evaporation temperature to 70 °C. The top-flow liquid in the two-effect evaporator is the HCl + H2O water vapor at the azeotropic point, which is an aqueous solution after cooling. It passes through a dilute acid cooler on the way, and the hydrochloric acid aqueous solution with a mass fraction of < 2% is obtained after cooling at the top of the tower. The cooled dilute acid enters a non-condensable gas condenser to cool and purify the non-condensable gas, and the dilute acid after purifying the non-condensable gas returns to the dilute acid tank; The bottom-flow liquid undergoes multiple forced circulations in the evaporation unit and completes evaporation and concentration. After cooling at the bottom of the tower after concentration, concentrated hydrochloric acid with a hydrogen chloride concentration of 21.69% is obtained;

[0056] (4)The concentrated hydrochloric acid with a concentration of 21.69% obtained in step (3) is recycled to the process of treating fly ash or gangue through a circulating acid pump.

[0057] The acid leaching solution of gangue obtained in step (1) of this example is 100 L, and the finally obtained concentrated hydrochloric acid with a concentration of 21.69% is 45.7 L.

[0058] The content of free acid and metal oxides at each stage during the sample treatment process is shown in Table 3.

[0059] Table 3

[0060]

[0061] After calculation, the recovery rate of HCl in the concentrated acid is 88.4%.

[0062] Comparative Example 1

[0063] This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution. The difference from Example 1 is that in step (2), the acid leaching solution does not add ammonium chloride, sodium chloride, and EDTA and directly enters the primary preheater.

[0064] The fly ash acid leaching solution obtained in step (1) of this comparative example is 100 L, and finally 56.6 L of concentrated hydrochloric acid with a concentration of 11.30% is obtained.

[0065] The content of free acid and metal oxides at each stage during the sample treatment process is shown in Table 4.

[0066] Table 4

[0067]

[0068] After calculation, the recovery rate of HCl in the concentrated acid is 75.4%.

[0069] Comparative Example 2

[0070] This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution. The difference from Example 1 is that ammonium sulfate, sodium sulfate, and citric acid are used to replace ammonium chloride, sodium chloride, and EDTA in step (2).

[0071] The fly ash acid leaching solution obtained in step (1) of this comparative example is 100 L, and finally 54.5 L of concentrated hydrochloric acid with a concentration of 14.37% is obtained.

[0072] The content of free acid and metal oxides at each stage during the sample treatment process is shown in Table 5.

[0073] Table 5

[0074]

[0075] After calculation, the recovery rate of HCl in the concentrated acid is 77.4%.

[0076] Comparative Example 3

[0077] This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution. The difference from Example 1 is that in step (2), the preheated mixed acid leaching solution is introduced into the first-effect negative pressure evaporation system at a speed of 40 L / min, the vacuum degree is set to 25 KPa, and the evaporation temperature is 40 °C.

[0078] The fly ash acid leaching solution obtained in step (1) of this comparative example is 100 L, and finally 50.2 L of concentrated hydrochloric acid with a concentration of 19.26% is obtained.

[0079] The free acid and metal oxide contents at each stage during the sample treatment process are shown in Table 6.

[0080] Table 6

[0081]

[0082] After calculation, the recovery rate of HCl in the concentrated acid is 81.8%.

[0083] Comparative Example 4

[0084] This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution. The difference from Example 1 is that in step (2), the preheated mixed acid leaching solution is introduced into a first-effect negative pressure evaporation system at a speed of 60 L / min.

[0085] The fly ash acid leaching solution obtained in step (1) of this comparative example is 100 L, and finally 47.3 L of concentrated hydrochloric acid with a concentration of 19.23% is obtained.

[0086] The free acid and metal oxide contents at each stage during the sample treatment process are shown in Table 7.

[0087] Table 7

[0088]

[0089] After calculation, the recovery rate of HCl in the concentrated acid is 83.0%.

[0090] Comparative Example 5

[0091] This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution. The difference from Example 1 is that in step (3), the separated upper-layer HCl + H2O mixed water vapor is introduced into a second-effect negative pressure evaporation system for evaporation and concentration at a flow rate of 30 L / min, with the vacuum degree set at 5 KPa and the evaporation temperature at 90 °C.

[0092] The fly ash acid leaching solution obtained in step (1) of this comparative example is 100 L, and finally 52.8 L of concentrated hydrochloric acid with a concentration of 14.76% is obtained.

[0093] The free acid and metal oxide contents at each stage during the sample treatment process are shown in Table 8.

[0094] Table 8

[0095]

[0096] After calculation, the recovery rate of HCl in the concentrated acid is 80.5%.

[0097] Comparative Example 6

[0098] This comparative example provides a method for recovering and recycling free acid in fly ash acid leaching solution. The difference from Example 1 is that in step (3), the separated upper-layer HCl + H2O mixed water vapor is introduced into a double-effect negative-pressure evaporation system for evaporation and concentration at a flow rate of 10 L / min, with the vacuum degree set at 20 KPa and the evaporation temperature at 50 °C.

[0099] The fly ash acid leaching solution obtained in step (1) of this comparative example is 100 L, and finally 57.1 L of concentrated hydrochloric acid with a concentration of 13.35% is obtained.

[0100] The content of free acid and metal oxides at each stage during the sample treatment process is shown in Table 9.

[0101] Table 9

[0102]

[0103] After calculation, the recovery rate of HCl in the concentrated acid is 84.9%.

[0104] From the above results, it can be seen that the method of the present invention significantly improves the acid recovery rate, optimizes the heat energy utilization at the same time, and reduces the energy consumption. In Comparative Example 1, the mixture of ammonium chloride, sodium chloride and EDTA was not added to the acid leaching solution. The lack of these enhancers led to poor acid volatility, large acid loss during evaporation, 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 replaced composition did not sufficiently reduce the consumption of acid by metal ions and could not effectively complex metal ions, resulting in large acid loss. Comparative Example 3 adopted a lower evaporation temperature and a higher vacuum degree, and the evaporation process efficiency was low, and the acid volatilized incompletely. Comparative Example 4 adopted a higher evaporation rate during the evaporation process, which might cause part of the acid to volatilize too fast and be difficult to recover. Comparative Example 5 adopted too high an evaporation temperature, resulting in too fast acid volatilization and less recovered acid. The evaporation speed of Comparative Example 6 was too slow, resulting in insufficient processing capacity of the system, a relatively slow evaporation process, and a relatively low acid recovery rate.

[0105] The above is the preferred implementation manner of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle described in the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope 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; In addition, 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; 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; 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; In step (2), the mixed acid leaching solution 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; 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.

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

4. 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.

5. 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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